Same thing happened to me when I first started charting and came off the pill. I was waiting for AF to return after stopping the pill, about 4weeks later I was getting AF cramps followed by AF a week later. My AF was the same as when I was on the pill, lasted 3 days, but fairly light. 3 weeks later I was getting cramping again and thought I was close to O'ing, I was getting +ve OPK for a week and then did a HPT which was also +ve. During all this time I never had a clear thermal shift, it was always quite high. When I had BT it said I was 5-9 weeks along, I declined a dating scan, as I knew I was 7 weeks.
I suggest a HPT, good luck.