I was diagnosed with Ankoylosing Spondylitis 6 years ago. About 3 months ago I started having pain in my thighs at night, if I lay on my left side the pain is in my right thigh and vice versa. It goes from the top of my thigh down to top of my knee and is mostly on the side to the back of my thigh. The pain wakes me up at night and once I lay flat on my back it starts to lessen. Any ideas as to what's causing this? Thanks much.