Had a 12 day period!!! What in the world?

We have been TTC for about 6 months now with no luck. In July, I had half-dollar sized clots FALLING out of me. Since then, I've had terrible menstrual pain at the start of every period. I've seen an OBGYN who told me nothing but "Your blood is supposed to clot" and "Your cramping could just be your period changing". Well, I've scheduled a second opinion and have an ultrasound scheduled for Sunday along with an endometrial biopsy. Now, this month my period lasted 12 days! It usually only lasts 7, TOPS. Anyone had similar situations? A lot of people are telling me it sounds like endometriosis... but of course they aren't doctors.