irregular periods

Has anyone else experience irregular and sometimes absent periods after a baby when they were always spot on before baby? I was always to the day before my son. .... and I'm not talking shortly after having a baby.... I'm talking like my son is 18 months old, I haven't breast fed him in nine months. I didn't have a period for 8 months unless it was medically induced (provera) then the 9th month it started on its own, and I had one in Jan, but none in February again and still no period.... I've taken two at home preg tests and both said negative. Has anyone else ever experienced this sort of thing? I've also had a million tests ran for pcos, auto immune, and on and on and all came back negative.