What should I do about this crazy girl?

This girl is really weird and seems to lie about really important stuff. I thought I liked her but not now. So the whole story is I started to like her we did stuff and ended up having sex. She told me she was on birth control and a condom she provided. Then like five days later says I haven't had my period yet and I go you were on birth control and she goes I lied about that. I was pissed that she did that! Then I found a good friend of hers and asked if she knew when she had her period and she said in the middle of the month. Now she's lying about missing it too. Then she tells me she has a boyfriend too. I'm about fed up with her crap would it be OK just to let her do whatever and if she's pregnant just not care because of her crap?