your tendons in your hip are probably tight. Working out more will strengthen them and stretching will allow them to relax! Take it from someone with hip issues :) and also use a heat pack on occasion when it is really sore, it will relax the tendons and muscles enough to stop pulling on the joint/bone so much so you can relax. And advil. Take an advil!
I feel it worst when I am driving, or when I lay on that hip. Riding makes it feel better lol. But for some reason, I just can't open my left leg all the way without feeling it right on the inside of the joint. You are probably right with it being the tendon, and normally, if it were any other ache, I would just "pull it out" to stretch it, but I can't do that with this one.
It started after sitting through a spook, I caught myself hard on my left leg, probably from bracing in the stirrup instead of relaxing (what spooked the horse, spooked me too,
something slithering in the bushes off the trail). I can post a trot, sit a trot, stand or sit a canter, and I'm not in any discomfort. I don't even remember it until I sit in a chair or car seat in a certain position.
I don't take aspirins if I can help it, but I will have to see if a heat pack will work.