late period and not pregnant

Hello. On my day 41 of my cycle and haven’t had my period. Took multiple pregnancy tests and they are negative. Went to the doctor and they said it’s because I have gained weight- 15 lbs. I just don’t think that it’s the reason. Has anyone else had late period and not been pregnant? My periods were a 30 day cycle to a T and now it’s been 39 day cycle starting in October and this month it’s been 42 day cycle w no period. I’m getting worried