Hmmm. When you say hip joint....is it actually that...where the femoral head fits into the acetabulum?
Or perhaps is it more like down lower where the greater trochanter is? If that is the case it MAY be a trochanteric bursitis. That hurts like all get out.
Have you changed shoes lately? Higher or lower heels? Different
gait? Have you done a great deal more walking than usual?
Are you sitting any differently lately like if you carry a wallet in your back pocket and it throws you to one side when sitting?
If you are having "sciatica" down your leg on the outside it would point more to a L5-S1 disk problem. That is felt kind of behind the acetabular area and down the "sciatic notch."
Does your mattress need turning? Don't do this yourself on the off chance it is a disk.
Legg-Perthes is kind of a far out dx I agree. Probably the first thing that your doctor thought of without thinking
Try lying on your back in bed, head slightly elevated and your hips and knees at 90 degree angles. It's a "Paul Williams position." Also known as semi fowlers with leg elevation.. You can use a stack of quilts or blankets to obtain the 90 degree leg flex.
I hope it is none of the above and you just had a bad night and woke up with a temporary problem.
Good luck. Let me know if you need more references.