Only 4 periods after stopping the pill??

I have only had 4 periods since stopping my birth control pills in January. Last august /September I all of a sudden didn't get a period at all and I was always regular. So the doctor put me on the pill to start it and regulate it. I took the pill from last September until the end of January this year. I got my first period in April and I also got them May, June, and July all on time like clockwork. But this month I am 3 weeks late on my period with negative tests! I don't understand. Happen to anyone else? Or any thoughts???