And you've already had an ultrasound that's shown baby is where it should be (not ectopic/in tubes?) There really isn't any vital organ in that area that would be cause to worry... kidneys are in your back, appendix on your right... it could possibly be a bladder infection. If there is no fever, chills or bleeding than you should be fine. I think the worst possibility would be a placental abruption, which would be very severe pain that doesn't go away. Or it could even be a sign of preeclampsia and if you're 19 weeks, that is around the right time. I'm not trying to freak you out, but there is no way any of us can diagnose you so it's ALWAYS better to make that phone call and be safe. That's what your doctor is there for, no problem or worry is too small!
Sounds like round ligament pain. mine hurt for like 2 days but it was on my right aise and my doctor told me to take tylenol and put a heating pad on it!