Is Pregnancy Possible While On Contraceptive Pills?

I have sex 3 weeks ago, I started spotting a week after. It gets on my underwear. It was unprotected but I am on birth control I have been for two years. I don't believe he came inside of me, but I am still nervous. I have taken 3 pregnancy test, all negative. I have been under a lot of stress and am stressing myself out even more. Also, I am only on my second sugar pill. So I may just be freaking my self out.