Stomach pulsing, is it normal?!

I've been on birth control pills for a year, always take them on time and didn't have anything like antibiotics to interfere with them lately. Tomorrow I should get my "period" and I've been feeling a constant pulse in my lower stomach all day. I've read that this is common in early or even later pregnancy. Now I'm kind of freaking out since I haven't noticed it before. Do you get it, is it normal?