It could be a muscle spasam. Do you have pain in your arms, legs, buttock or feet? Is the pain just in the middle or is it like over to the right or left side? If its more like the side but alittle lower than the middle of back then it could be a kidney stone that is irritated. Have you ever passed any? If not, then its probably related to nerve compression or overworking the muscles.. Have you recently picked up anything heavy or did any twisting while bending over?