Hello. I Am A 42 Year Old Black Female. Healthy,

Hello. I am a 42 year old black female. Healthy, 4 kids. Last pregnancy was 2012 multiples. No complications. I had my tubes tied and now I have periods sporadically. I have notified my GYN doctors twice I don't think this is normal, but they don't seem to see a problem with it. As far as I know, Ia m not pregnant. Does this happens sometimes'