What Causes Pain In Buttocks Post Knee Replacement Surgery?

I have had both knees replaced. My right one March 2011; my left 2014.
About 9 months ago my right buttock began hurting..sometimes it would cramp up and I'd have to sit down. About 6 months ago my whole right side began to have pain...sharp, dull achy, and I began limping because I felt that I could not put much weight on my rt. leg. There's not a moment when I don't have pain....sometimes not too bad other times a 10.
I have had X-rays of my kness, an MRI of my back with the orthopedic dr. I also got a 2nd opinion. I
Recently I had an EMG and results were normal. I still cannot walk properly, I get very stiff when I sit, or stand too long. I use a cane to help. I am taking a prescribed anti inflammatory med and hydro codon 10-325
I do have osteoporosis(do take mods) and some curvature of the spine.
I am under the care of orthopedic and neurosurgeon. I am not getting any better and feel I'm getting worse.