Braxton hicks?

So I ate pizza last night with my family and I was the only one who ended up sick. So after having vomiting and diarrhea this morning I still feel queasy and tried to take a bath to relax. I should point out that I have not thrown up yet this pregnancy and am currently 18 weeks along. So in the bath I felt like my stomach was tight and balled up in the front. Are these my first Braxton hicks? What do they feel like and when can they start?