Hello, I was on dianette bcp for two months starting in july this year (having previously used another bcp with bad side effects)

I took the pill in augast and everything was fine , however my first period was exactly one week late it was completely normal, my next period was also fine but it coincided with a UTI.
I decided to come off this pill in mid september as it did not agree with me either, I was due a period mid october but I am currently 3.5 weeks late. Now I HAVE NOT been sexually active in 14 weeks and I am concerned, I am being investigated for IBS and chrones as well. I have taken two HPT and had one at my local dr surgery and all of which were negative. My last HPT was taken last week and it was also negative. Any ideas as to why my period is late?