I'd say the boyfriend is lying. Not to make you feel bad, but it could be an indication that you're not as satisfying as someone else, he was thinking of her during the time to try to get into it. I know it happened after sex, but just cause you just finished sex doesn't mean that you suddenly start tinking about mowing the lawn.
It could be far fetched, because I don't know him, and he could just suck at knowing when to talk about stuff, but it's something I'd at least keep an eye on.
With sex, everyone fantasizes. Everyone does it differently to get the most pleasure out of it. Like I said, not trying to make you feel bad or paranoid, I just don't believe in coincidence so that's why I say it was one of those two things. Either way, it would be a problem for me if my girlfriend did that. It'd mean I was wasting my time with her or she just wasn't very bright, and it was still a waste of time. Finding out which one it is is the hard part.