What Is The Treatment For Irregular Menstruation?

i am 37 years old. i have two children. no abortions, still births, or deaths. my periods are sporatic. sometimes lasting up to 15 days. sometimes starting 5 days after stopping. example: my period stopped oct 26 and started again nov 5. my tubes were tied in 1996. sometimes the blood is black and sometimes bright red. sometimes they are so many clots i have to change very often. i saw my obgyn and he did an ultrasound. he didnt find anything. no cysts, tumors, etc. he said it might be stress. hmmmm?