My boyfriend and I are having problems he had unprotected sex with another girl and then told her he didn't want to be with her and he said they did it that night and she came on her period that morning and now she's claming she's 3 weeks pregnant is it really possible that she's pregnant or she just messing with his mind.

It's possible. Some women ovulate during their period.

The only way to know for sure is to wait for the child to be born and get a DNA test.

Yes, she could be, but it's kind of too early to tell. If she misses the next period or two, she should start learning lullabyes.

Does everyone sleep through their sex ed classes these days?