Anyone else???

My last period ended the end of May.. So almost two months with out a period. I have been feeling weird lately, so I had went to the free pregnancy clinic near my house and the test came back negative. Yet I still haven't had my period. It's been slightly late before, but usually if I have sex or masterbate it comes shortly after, and I STILL haven't had any sign of my period. Is it stress or should it be something I should worry more about. Anyone else have similar problems?