Scarleteen

I've been sexually active with my girlfriend for 6 months and have only used condoms for birth control in combination with withdrawal before I finish. I still get paranoid each month that she may have gotten pregnant. The other day, we had sex and I pulled out before I ejaculated and everything was fine. A few minutes later we had sex again with a new condom and I did not ejaculate. When I took it out how ever, the condom had moved up my penis a little bit and there was what looked like a bit of semen in the tip, the condom did not break or tear but I'm worried that maybe so semen came out from the previous ejaculatjon and got pushed up and out of the base of the condom. Is this possible even though it did break or slip off? Is there a possibility she could be pregnant?

As long as you don't see rips or tears, and the condom hasn't come off your penis (some sliding up, as you describe here, is ok) then you can assume the condom worked effectively.
It sounds like using condoms doesn't feel effective enough for you to really be comfortable right now, though. It might help to read over this article about proper condom use, to make sure you're hitting all the buttons when it comes to using them correctly for maximum effectiveness: Condom Basics: A User's Manual
If you are still feeling really worried about pregnancy every month, it sounds like it might be a good idea to either talk to your partner about adding another form of birth control in addition to condoms, or to hold off on intercourse for a bit, so you're not feeling so worried every month.