Okay, kind of a long story so brace yourselves.

I was on birth control pills for 10 months then stopped last July because I was becoming too moody and irritable. Five weeks ago, after being pill-less for one month, my first natural period started. Two weeks after that I started birth control pills again. Last week I was supposed to have my next period but didn't because of the pill. However, I still felt a few pms symptoms such as a little cramping and tender breasts then. This morning I woke up and noticed that my nipples looked larger (but not darker). I read online that pregnancy causes larger nipples, but have found nothing else to be the cause. Are my nipples larger because I'm pregnant? Or is there possibly another reason?

I am sexually active and use condoms each time.

As long as you are sexually active, there is a chance for pregnancy. Take a HPT to find out -- the hormone fluctuations you're noticing from on/off pill use could also be a factor.