It is highly likely that the "force" meant Eric raped Eva. The Inspector says to Eric that he "just used her for the end of one of his drunken evenings".
Eric also admitted that Eva was also slightly drunk because she had nothing to eat meaning maybe she wasn't conscious enough to make up her mind.
However it is hinted that Eva was a prostitute as the place she was most commonly found in was the Palace Bar home to the "women of the town" (Prostitutes). When Eva was pregnant she couldn't tell Mrs Birling that she was raped by Eric because at the time people wouldn't take the thought of a prostitute being raped seriously.
This is why Eva couldn't possibly stop Eric coming into her apartment or why she would still see him after the the events that took place simply because of her role in society, this is one of the major reasons Eva commits suicide as she was constantly treated as garbage by others due to her "class" and exploited for her "pretty" looks.