I went back and watched the start of this thing tonight to see what made Eva leave. It looks like she went into the bathroom with Sam to tell him that she didn't want to go through with it tonight. Sam probably already popped a blue pill and said, sorry but it's going down with or without you. She stayed for awhile, then abruptly, but quietly left the apartment. I don't think she was going to meet anyone, I just don't think she wanted to hear what was going on in the bedroom.
Whole lot of speculation and filling in the blanks there of course, but if I'm right about her wanting to call it off, Sam fucked up by going through with it without her consent.