What Causes Delay In Menstruation Other Than Pregnancy?

I stopped taking BC pills in on June 15, I had a period June, July and Aug. I was suppose to satrt Sept 24-29(4-5 day variation) and it is now Oct 17, and still no period. I took two hpt all neg. my doc took urine test and neg. Still no period, I have also been experiencing lower ab cramping like my period was coming, but no period, and I dont usually cramp with my period only sometimes right before it comes down, no tender breast or any other sign of pregnanacy besides the missed period. I called my doc again and she seems to not be worried and says wait a few months or I could get my thyroid checked. I had my thyroid checked in Feb, when I had a physical and ill my blood work was normal. Could I still be pregnant with all the neg reads on urine test? This past Friday made it three weeks from when my period should have began.