Girlfriend says she is pregnant but i think she cheated and its another guys baby.....?

I am only 17 years old and my girlfriend told me today that she is pregnant and its mind but i don't think its mine because when i was 13 i had leukemia and was told i wouldn't be able to have kids because the chemo made me sterile. Well i don't know what to do my girlfriend doesn't know and so its like i think she is cheating on me since i can't have kids! What should i do? Advice?