The coccyx.blunomore, try this:
Lie on your back with head on a thin pillow. Knees pointing skywards. Now suck your stomach in and also that weird sharp bone where the spine ends (forgot the name). Keep that position for like 5 mins. (Don't forget to breath)
If it doesn't improve with that stunt (Don't think it will tbh) then its unlikely to be muscles/back but rather kidneys. In that case, drink loads of water and lose the coffee/alcohol for a few days. And go easy on the painkillers...they only make the actual problem worse. The fact that voltaren isn't working also makes a muscle issue unlikely.
I use Reparil and it works better than Voltaren.
I once pulled an inner thigh muscle and applied Deep Heat. Needless to say, I wasn't crying because i rubbed my eyes.Lol. I once applied Deep Heat, and then scratched my eyes..
My friend (chick) at the time though I was crying...
Now that I think about it, I might have been....