Late period

Hi I’m contacting the doctor tomorrow but wondered if anyone had any idea what is going on? I had my period like usual last month but this month it is 13 days late and I’ve been having a funny feeling in my belly (like bubbles and a heavy feeling) as if I were pregnant. I’ve taken 2 different brands of tests (including clear blue) with days in between them and both were negative. Any ideas why this could be happening?