Man, 21" top tubes are considered long enough in the bmx world so it ain't too good to be having problems on one. Have you like just literally started riding around on it from a MTB? Or have you been on it (and it alone) for a fair while? If you changed over from a MTB then it's obvious you need time to let your body adjust to the new position it'll be in for riding bmx. It took me like at least a month before the back pains went (although mine weren't as bad as you make yours out to be...).
But anyway, try to avoid long ass frames. Obviously if your back is going to f**k up if you carry on then go for the 22" or whatever, but it's generally a lot harder to pull stuff on longer frames. So, erm, good luck anyway.