What Are The Chances Of Pregnancy After Having Protected Sex?

If I am on the pill and my boyfriends uses a condom, how likely am I to be pregnant? Im just paranoid all the time instead of enjoying it.

Yesterday I had a sudden cough all day n bad headache, now today I have had lower back pain all day. I also had a a cold for 2 weeks thats I have only just fot rid of. Are these symptoms of pregnancy or something unrelated and just a cold and usual back pain? Please help!