Sonnet 90 continues from sonnet 89, hence the "Then" at the beginning of the poem. These sonnets are part of a sequence (sonnets 88 - 93) about fear of losing the friend's love.
In sonnet 89, the poet says to the friend: "If you deserted me because of a fault (or worse) on my part, I would support your arguments against me, since I cannot love someone that you hate."
Sonnet 89's last word, "hate", returns as sonnet 90's second word. Line 2 says that the entire world is determined to thwart his actions. It is not clear whether this is hyperbole (the friend's rejection is inflated to a general conspiracy) or whether the poet/narrator is also experience problems that are unrelated to the relationship. (The verb "join" in line 3 supports the latter interpretation.) Either way, if the break in their relationship is to come, it should come now rather than later ("if ever, now" from line 1). Loosing the friend is the loss that is the sonnet's subject.
"After-loss" (line 4) is not used anywhere else in Shakespeare, but is is formed in a similar way to a number of other words in Shakespeare:
- after-debt ("He nere payes after-debts, take it before", All's Well That Ends Well, Act IV, Scene 3): probably a debt to be paid at a later time, outstanding debt;
- after-dinner: ("an after-dinner's sleep", Measure for Measure, Act III, Scene 1);
- after-inquiry: ("jump the after-inquiry on your own peril", Cymbeline, Act V, Scene 4): subsequent investigation.
So after-loss most likely means "something that comes after a loss", in this sonnet, after the loss of the friend's love.
Drop in means "fall upon", here in a figurative sense. Another instance of "drop in" in the sense of "fall upon" (in a literal sense) can be found in Antony and Cleopatra, Act III, Scene 13:
Ah, dear, if I be so,
From my cold heart let heaven engender hail,
And poison it in the source; and the first stone
Drop in my neck: as it determines, so
Dissolve my life!
For does here not have the common modern meaning indicating purpose, as "You came for gold, ye slaves." (Timon of Athens, Act V, scene 1). "For" here mean "as; by way of". Here are a few examples of this usage elsewhere in Shakespeare:
So line 4 means "do not fall upon me (figuratively), thereby adding to the loss". As mentioned before, line 1 says that the poet prefers the shorter pain ("if ever, now"). In addition, lines 5-8 continue this thought, using other words: do not persecute me after ("in the rearward") of my current distress, do not come like a rainy morning after a windy night, to protract the destruction.
According to John Kerrigan's thoroughly annotated edition of The Sonnets and A Lover's Complaint (The New Penguin Shakespeare, 1986), drop in for an after-loss means "fall upon me, as a belated loss" (page 286).
The received interpretation of drop in, 'call in casually', relies on usage not current until a century later, and is bathetic.
Note: Reading "drop in" as "call in casually" is bathetic because its banality stands in stark contrast to the high poetic style of Shakespeare's sonnets. (That alone should have alerted any astute reader of poetry.) Colin Burrow, editor of The Complete Sonnets and Poems in The Oxford Shakespeare (Oxford University Press, 2002) agrees with this reading.
A response to Peter Shor's interpretation
Peter Shor found an occurrence of "dropping in" that, according to him, may have the meaning "to make an appearance on the scene", namely in Of the Head-Corner-Stone by Edm[und] Bunny (1611).
Below is part of the context for "dropping in" in Bunny's Of the Head-Corner-Stone (my emphasis):
As touching those that did succeede, although they are no part of this people, but meere strangers vnto them, and at this time might rather be held to bee of the number of their enemies also: yet because they goe oft-times vnder the names of Israelites, and not vnlikely, but that diuers of the Israelites (such as shog'd aside for the time, to auoide the danger of the enemies Sword, and afterward came dropping in againe) did afterward come in and dwell among them, at least because these doe now hence-forward inhabit that Land where the Israelists dwelt before, therefore it shall bee good to see the Story of them.
- The interpretation "to make an appearance on the scene" is vague. The context of this instance of "[came] dropping in" in Of the Head-Corner-Stone is a commentary on 2 Chronicles 30, 1-13, where "Hezekiah sent word to all Israel and Judah and also wrote letters to Ephraim and Manasseh, inviting them to come to the temple of the Lord in Jerusalem and celebrate the Passover to the Lord, the God of Israel." The people who "came dropping in" were people who had moved outside Israel and came back.
- Of the Head-Corner-Stone was printed in 1611 by William Jaggard, who also printed many of Shakespeare's works, including the False Folio (1619) and the First Folio (1621). Shakespeare's sonnets were printed just two years earlier (1609, by a different printer) but probably mostly written in the 1590s. Peter Shor claims in a comment, "If a usage was around in 1611, I don't see why it's unlikely to have been around in the 1590s." This ignores that fact Early Modern English developed in an era of great linguistic change, so words and meanings could come into fashion and disappear again fairly quickly. From the 1590s to 1611 is long enough for new words and meanings to be added to the language.
- Finally, as I mentioned before, the interpretation "visit informally" (which clearly does not apply to the above passage from Bunny's book) is simply to mundane for the context of Shakespeare's sonnet. Readers who do not understand this should (1) read much more English Renaissance literature and (2) learn to recognise bathos. For examples of bathos, see Encyclopaedia Britannica, ThoughtCo, LiteraryDevices.com, LiteraryDevices.net and LiteraryTerms.net.