Psoas (soass) muscle, I had this pain years ago so bad I was off work, reoccured recently & Osteopath told me that's what it is (I'd never heard of it). I was fobbed off years ago following xrays, nhs physio, strong painkillers etc, it can be excruciating at times. She has given me two exercises (it is in my left groin), lie on your back, bend my left knee, keep right one straight, as your knee is bent let it go out to the side and back up again x3 then slide your leg down so it's flat and slide it back up again, repeat this x5. The exercise she gave me this week is again on your back right knee bent this time, left leg hanging off side of the bed rotate your leg x10 one way then x10 the other, most painful for me when I do it clockwise. It's a really deep pain, makes me waddle when it flares up, recommended to use either hot/cold pack on it to help reduce the pain, also taking diclofenac and paracetamol befor ebed as that is when it is particularly bad. Hope you get some relief Paz23. x