What Are The Side Effects Of Using Emergency Contraceptive Pills?

I have been on and off birth control a couple of times, I restarted my birthcontrol about three months ago,I've never had any problems with it period was always normal, Now this is my third "period" of brown spotting, extremely light and no blood, I've never had this before. I do have unprotected sex and missed a few pills last month, and was having unprotected sex when i first started the birth control. I have been nausea's and have had a few throwing up spells, but my boobs don't hurt. I'm really confused, please help! could i be pregnant?