Condoms are not 100% effective in preventing pregnancies or STDs. Neither is the pill or any other form of birth control outside of abstaining from the act. A combination of these things will help, but again it's not 100% and some couples find that out the hard way.
There is the "morning after" pill (aka "Plan B"), but from what I understand, it has to be taken within so many hours (or days?) of having intercourse. I think most states have a policy in which the girl/woman can go to a pharmacy and ask for it without a prescription, but you'd have to check on that yourself. It doesn't guarantee to "abort" a potential fetus, but it raises the odds of one not becoming viable....
I don't think you have anything to worry about tho'. If you're really concerned, get an early home pregnancy test. Wouldn't recommend it every month tho'!
Also, I'm with lum510 in the dept of "if you're not ready for a kid - don't have sex!" matter. Having sex with someone and getting pregnant is not just the girls responsibility - that belongs to BOTH partners equally! Once it becomes a viable pregnancy your limited on what can be done, so be prepared for the possible consequences!! Not ready for them? Don't have sex.
NO there is no pills to abort a child! If she is pregnant she cant abort a child with a pill or doing something other than surg. dont let anyone tell you diffrent. things like this are very serious and doing some thing stupid could hurt your girlfriend. But 1st things 1st has she taken a preg.test if not she should take one at least 3 days after her missed period. If neg. and she still doesnt start taken 2nd one at least 7 days after missed period. Then if not started yet she should call her Doctor to set up an appt. they can then run other test. But I dont think you have any thing to worry about.
But please let me say this if you are that worried about having a baby may be you should not have sex until you are ready for a child to be brought in to the world. A child is not just something you can make go away with a pill or something it is a life!
Please be careful and think about your actions. Things like this will change your life forever. Be safe and good luck!