So sorry about the pain and misery you are going through. I just wanted to discuss what my experiences have been with this issue. I am much older than you (52) and have dealt with horrible menstrual cycles for many years. My symptoms pretty much started from the very beginning as a teen. So I have had plenty of work-ups ordered by my ob/gyns throughout the years. Diarrhea can, and often does, occur during your cycle. It has something to do with hormones, as well as uterine contractions and the inflammatory "prostaglandins" that are released at that time. It tends to co-exist with dysmenorrhea (which is a term for painful menstruation) and in is often diagnosed as IBS, because it has the same physiology.
From my understanding and personal experiences, endometriosis cannot be diagnosed by ultrasound or any tests, other than surgically. Uterine fibroids can be viewed by ultrasound. I was diagnosed with "likely" endometriosis for many years, until it was confirmed by a total abdominal hysterectomy only 5 years ago. Uterine fibroids were viewed and diagnosed by ultrasound a few years prior. I also had ovarian cysts for most of my adult life. Severe cramping, heavy bleeding, and yes, diarrhea were all symptoms of my menstrual cycles. It was absolutely miserable and I can certainly sympathize with you! I'm not saying hysterectomy is the answer or cure. That was just the option I chose because I was 48 yrs old and it was right for me.
Several years ago, an ob/gyn told me I had IBS. Her exact words were "People that have Type-A personalities seem to have IBS, endometriosis and other things, including fibromyalgia..." She told me that none of those conditions caused the other, but that they just tend to co-exist or correlate. She put me on a high-fiber diet to help with IBS symptoms, Prozac and told me to take ibuprofen 800mg twice daily and start taking the ibuprofen several days to a week before my period was to start. Other than that, I usually took Immodium for the diarrhea and used a heating pad.
Again, this is not medical advice but just my own personal experiences. Continue working with your ob/gyn. She may try different types of bc pills and I have even heard that the IUD called Mirena can help...among many other treatments. So good luck to you and take care!
DDD, 3 bulging lumbar discs, bone spurs, facet joint arthrosis/sclerosis, osteoarthritis, possible fibromyalgia (my dr's are telling me that I "most likely" have fibro but won't actually diagnose it...)