Um so yeah, 3-4 sex partners...really...
You use condoms, do they?
Can this junky slut prove it's your kid even?
Will she even be able to carry the baby full-term?
I know I sound cruel, heartless, etc., however, these are the real questions, and so logically, calculate out the odds that she is pregnant, with YOUR child, and how long it will last.
Oh and most pregnancy tests are accurate to 6 days within a missed period. you can get 2 or 3 different ones for less than $30, so it'll be worth it for peace of mind.
By the way, get yourself checked for STD's.
Oh and stop fucking her. It'll be the best thing to happen to either of you.