I'm in a similar situation and actually have an artificial hip. (Bermingham Hip Resurfacing) Like you my hips are really tight and I'm almost 2 years post surgery and still don't have anywhere near the strength I once had nor the flexibility. I'm working on it though and know I have a disadvantage so I keep that in consideration. My surgeon said for hips that the leg press is the king of all exercises and he recommends those over squats all day long. Maybe you just need to go light for a while and see if that might help? The heavy weight could easily but what's causing the issue. When overstressed the joint will get inflamed which could be causing part of your flexibility issue.