I've had on and off lower to middle back pain for more than a month... when it's on it also makes my butt feel a bit numb. Heat and/or a Ben Gay type ointment makes it feel better. What could be causing it and how do I make it feel better?

Sounds like you may have a pinched nerve and BenGay isn't going to help with that. A medical doctor should be able to help but I'd go to a chiropractor.