Very painful periods can be caused by endometriosis and that will not show up on the ultrasound. It can be diagnosed only by doing surgery. Google it and see if your symptoms might be caused by endo. I have it and I know how miserable one can be, but there are solutions. Your doctor can certainly tell you more. And heating pad is very helpful with menstrual pain.
13gemini72 is right, endometriosis can cause very heavy periods and bad cramping. Also, cysts, no matter the size can cause pain. It is a wonder how something so small can cause pain, but it can. I'm glad to see that you are going to see your gynecologist. I know 3 weeks seems like forever away (I will have surgery in about 3 weeks), but maybe you could call your pcp and just tell them what your medical record shows and ask them what they think. I don't know what your pcp's office is like, but I can just call and ask our nurse questions like that. Just know that you will find amazing women on this forum who will do their best to support you and give you the best advice we can. Please let us know what you find out.