@ParanoidePatrick,
First of, let me start by saying I find it rather stupid to get involved with an 'easy' girl for sex, if you are afraid you might wind up being a father. This seems to generate a lot of stress for you, which you could avoid if you just took care to only have sexual intercourse with a committed, monogamous partner who uses protection herself in addition to the condom. Also, it doesn't seem particularly nice to label the girl as 'easy' , when you yourself don't seem to have any issues with sleeping with her: that makes you 'easy' as well, I'd say.
Second, afaik, I don't think you are the father. The math doesn't add up. I assume she doesn't check on a daily basis whether she is pregnant or not, but she found out after she missed her period. That means her ovulation was at least two weeks prior, plus the time that has elapsed since she missed her period, probably also 3 or 4 days. That means you are almost certainly not the papa of this little baby.
But if you really want to be sure, take a paternity test.
And if you want to continue having sex with 'easy' girls, but are overly worried about pregnancy, have a vasectomy. That should take care of all your worries.