That is the key element right there. OP, can you pinpoint the area that hurts? Meaning, can you find the area or spot with your finger and if you apply pressure does it feel like a really deep bruise. Sure, mobility issues and tight piriformis can wreak havoc on your legs and hip flexors and that may be your issue, it may not.
I have, and continue to have issues, in both hips. It is right at the anterior joint directly on top of the done. TFL area. My mobility is pretty good, actually very good compared to a few years ago. One thing you can do to help give a better idea of the affected area is this. Sit in a chair with you but close to the edge of it. Lean forward like your going to stand up and as soon as you engage your glutes, quads and hips to get your but off the chair do you feel the pain in that hip. What I am getting at is it could be a muscle, or group of muscles (adductors) that for whatever reason has had to compensate for a weakness or imbalance somewhere else and the stress was just too much for it.
Go to a chiro that specializes in sports injuries, therapy, and is licensed to do ART. There are so many different possibilities beyond the cop out of arthritis (like my ortho told me and was wrong). The muscles in the legs take so much abuse especially when squatting and pressing heavy weights that they can become stuck or adhere to each other (not literally) and not move and glide over each other smoothly like they should, especially in the hip area. Good luck.