Hey everyone - I posted a topic about this a while back and had the same issue. So I will share my experience a little here hopefully it can help however it may.
After completing 28 sessions of chemoradiation, like you guys my hip area started to ache and feels very tight (but not painful, I also had slight backache. I wasn't able to do the usual stretches that I could. My radiation onocologist ruled out bone cancer first and foremost but he was nonchalant about the aching I was experiencing in my hip !
I engaged a trainer for some other reasons (to keep fit for surgery) and the trainer advised me some hip exercises when I raised the issue up - the leg extensions machine especially helps. She also recommended the footballer one-leg hip rotation exercises but in the water/pool for both sides of the hip.
It wasn't only very much later that my chemo onocologist told me that the bone marrow in our back is responsible for white blood cell hence when the body is under "attack" either by radiation or chemo, some of us can feel back pain. He didn't specifically mentioned hip pain /ache but I really believed that the back pain due to the bone marrow led to my hip ache.
After 2 months out from chemoradiation, it went away but then I was back on chemo and after 1.5 moths, the same hip ache is starting again. So i truly think that the low white blood cell we are experiencing could be the reason.