Hi Craig!
Welcome to OCF, glad you found us sorry you had to! It's nice to see a 5 year survivor join the forums, I'm sorry it had to be due to this issue!
Aches and pains are common after a mandiblectomy to be sure, even from the donor site. Sounds like a Psoas Major, one of the three hip flexor muscles, pull or strain to me. These can occur when the muscles are tight and unstretched. I'd see a Physical Therapist about it actually. Stretches and exercises should help muscle loosen up and ease pain.
Quick fact, the hormone response in the brain from physical exercise is as effective as antidepressants at treating depression, it also helps lessen pain due to brain chemistry's affect processing pain signals. You're brain feels more pain in a depressed state. Pain medications on the other hand over time cause depression...do you see the vicious circle opiates can cause which is why they are commonly abused by patients using them to manage chronic pain.
Hope that helps

Eric