PLS HALP! Undiagnosed Finger Injury PIP and PP

Hi MP, I'm wondering if someone here can help me out... I'm having difficulty getting a proper diagnosis of an injury to my left middle finger. I’ve had the injury for about 4 months now and I've seen a doctor and a hand therapist, and had xrays and an ultrasound but they haven't been able to work it out (no MRI yet).

The pain occurs on the sides of the PIP joint and on the back of the proximal phalanx. However, the only time I really feel the pain is:
1. when the finger is in partial flexure and loaded laterally (pain near the PIP joint); and,
2. when the finger is straight, and I use my other hand to flex it at the MCP joint at 90deg. (pain is right along the back of the PP).

The ultrasound showed that I have a small (3mm dia.) ganglion cyst on the flexor side of the PP near A3 pulley, but I've been told that that is unrelated to the pain. And the xray didn't show any fractures or joint thinning that might indicate arthritis.

A while back I hyper extended the finger back at the metacarpophalangeal joint, and I'm not sure if that's related.

Has anyone experienced anything similar or have any advice?