Twisted pelvis. Go and see a chiropractor for a body alignment check up.
I suffered many years ago, struggled to walk in the end, a mate told me to see a acupuncturist so I did. This guy was an ex semi-pro footballer and he spotted straight away as I lay flat, that my pelvis was out of line. So of to the chiro, 3 sessions in 2 weeks (to allow the muscle to re-adapt to the new position of the pelvis) problem fixed, 20 yrs later all's fine.
My G.P. at the time said that it was arthritis, the chiro believes that it was my karate practise where you kick into empty space, it knock the pelvis out, fast bowlers get it, golfer get it.