hard lump next to hip bone

I noticed a hard painful lump the size of a nickel right where the crease between thigh and torso meet but towards top of hip bone. It comes up and goes down. When it comes up its hard and painful. When its all the way down I can't feel it. Sometimes it stays in the middle. I had a transvaginal ultrasound but they couldn't see anything. I'm wondering if it's because my uterus and large cyst was in way. Hopefully an abdominal ultrasound will show something. Anyone have this before or know what it could be??