I had unprotected sex a few days after my last period about 3 times and I missed my period by a week and I had unprotected sex again last night I think I could be pregnant I've been feeling nausea and fatigue, but this morning I started bleeding and feeling a bit crampy with back pain and I still feel nauseous.. please help?

Any time you have unprotected sex you risk pregnancy. Now you are playing the waiting game. You need to wait to see if your next period comes. If not, be sure to test using first morning urine.