Im on seasonale birth control, i only have a period

Patient: Im on seasonale birth control, i only have a period every three months. I take my pill same time everyday. I had my period last week and started my pill back up on sunday. Yesterday, thursday, i had unprotected sex. Now today i have serious cramps, like period cramps. Whats going on. Should i take plan b. can anything happen in 24 hours. I was only back on the pill for four days, sticking to my normal cycle of pills.

Doctor: