Scared I'm pregnant!

I lost my virginity a few months ago to my awesome boyfriend. I've been on birth control since before we met for my periods, and he's always used withdrawal... But I'm terrified every month that I'm pregnant.
My breasts are enlarged and I'm using the bathroom very often. My period is due in a week. It's hard to tell when/if ovulation may have occurred because of the pills. I hope it's just PMS.
Any input from more experienced people is appreciated. This whole sexually-active thing is hardly worth the anxiety!
21-year-old college senior