I put a post on the old forum earlier this week with a similar problem, someone diagnosed it as piriformis - but it has moved more to my hip than in my bum, I am fortunate enough to have easy access to physio who worked on my lower back on both sides, just about waist level on both sides of the spine - he seems to think I may have jarred my back - I was in real agony on Monday and Tuesday - could hardly move. I have rested all week (partly because I had a minor op) and it does seem to have improved - try and do some back stretches and rest it and massage the area either side of the spine, hope this helps.
Whizzy