It's hard to say what it could be since the sonogram ruled out most everything. Sounds like a cyst or an ectopic pregnancy...but seems you'd have been able to see that. Also, weird, but I've read a lot of women have pain in the right ovary when the problem is actually in the left ovary, and vice-versa.
TMI maybe, but you've ruled out constipation, right?
Oh, also...the appendix is on the lower right side. You didn't have an elevated white blood cell count, did you? It could be appendicitis-- and if so, should be treated immediately. Appendicitis will hurt all the time, and not only when you press on it. Another way to tell is if you lift your left leg and try to hop on your right leg it will hurt really bad (and may impossible to do). Best wishes.