Weed and Birth Control

So I started taking birth control in February of this year. I began smoking with my husband about two months ago and have been getting serious cramps about two weeks before my period. This has never happened before smoking. When I was off the pill, I did cramp about two weeks before my period, that was normal at the time because I was ovulating during that week.

Am I ovulating even though we are trying to prevent pregnancy? Can THC stop the pill from working?