Severe Round ligament pain for 3months. Not pregnant. Ultra sound yesterday showed fibroids. Can fibroids cause RLP? Maybe endometriosis?

Yes. Fibroids can even involve the round ligament, although more likely to have exophytic fibroids growing away from uterus on the side of your pain. MRI shows fibroids to much better detail than ultrasound. Fibroid-related pain is typically worse right before or during menses. Consider consultation with experienced Interventional Radiologist who could discuss whether or not you're a candidate for UFE.
Yes… Pelvic pain is a common problem with many causes and should be discussed with a woman’s physician.