I think Atheist is being a bit harsh on you as you sound like you took the right precautions in the first place to prevent a pregnancy.
You also sound like you are quite young and want to make the right decision for both yourself and the baby (if there is one and its yours). I think that you best bet is, if she decides to keep it, ask for a DNA test to be done as soon as the baby is born. Do not wait until the child is a few months old, do it straight away.That way, you will know from the start if it is yours. If its not, you can walk away with a clear conscience. If it is yours, you can start to make the right financial contributions for the upkeep of the baby and if you change your mind about being in the baby's life, you can make it then.