I agree that you should get child protective services involved.
you can get dcfs involved. they will take the baby from her at birth if she is that bad and you can get the dna test done and take her with you. it may take a few weeks and the state will have her..... but its better then the baby being with the mother
Why is your gut telling you the baby is not yours? Does her story simply not add up?
I think this is serious enough that you should consider doing prenatal testing, despite the cost. Just don't fall into the trap of using one of the labs known for their inaccuracy. I'll send you a private message about the good labs (there are only two that I would recommend.)
Then after the baby is born, do a DNA test through the courts. (The prenatal tests are not certified for use by the courts to determine paternity.)
Regarding if she turns herself in for warrants before the baby is born, you need to talk to a lawyer about what it will take to get the baby if the baby is born in prison and also turns out to be yours.
If she is still using drugs, you really do have a lot of worries on your plate.
Here is a link about prenatal DNA testing:
http://www.nytimes.com/2012/06/20/health/paternity-blood-tests-that-work-early-in-a-pregnancy.html?pagewanted=all&_r=1&