Hip flexor tendinopathy.
Physio said I have very short hip flexors and advised daily stretching. Also single leg squats using a pole of some kind to balance. And a single leg glute bridge.
I've also been combining my achilles tendon heel drop exercises with one for my hip where I just stick the other leg out in front while I do them. This is supposed to be good for hips although that one wasn't from the physio, so who knows?
The combination of everything seems to be working, though!