2 1/2 weeks between periods

Today I started my period after my last finished only 2 1/2 weeks ago and I'm really confused. We had sex last weekend which was when I was supposed to be ovulating now I'm really confused. My periods have been coming less and less over the past few months but also have become much more painful than they used to, I am having really bad cramps and really intense painful lower back pain. For the past week I've also been feeling really sick and not being able to eat properly I keep feeling full and quesy but don't know why. Should I be worried??