What Are The Side Effects Of Emergency Contraceptive Pills?

Hello! Ok so I never been on here before but I wanted to ask and see if I was pregnant? My boyfriend and I had a lot of sex last week so it's already been a week and didn't wear a condom. I bearly started my birth control the day right before he came to visit. Now I'm starting to have lots of headaches and feel dizzy and my stomache hurts. I took a cheap pregnancy test yesterday and it showed negative but I still feel sick? I started my period today which is a dark brown color it's not like regular and it's not a heavy flow like a normal period. What do you think?