Not exactly back, but buttock pain.

Hi, I'm an 18 year old female and for about 3 years on and off I've been having stretches of time, sometimes months in a row, where i feel an intense cramp-like tight pain deep in my left buttock when I go to bend over or sit down and again when I straighten back up. It takes at least 10 seconds to go away and is excruciating. It definitely isn't in the hip joint or anything and feels like a muscle cramp. It began after a season of rowing, but if it was a muscle injury from physical activity, wouldn't it have healed by now? My doctor was baffled and just told me to eat more bananas, but I'm not a big fan of them and maybe I'm a little skeptical that a little fruit is going to make this go away, especially since its just this area, not my entire body, that cramps up.

Anybody out there a butt expert?