Why does my middle-upper back hurt?

I have had middle-upper back pain for a while. The is a tender, dull, ache right down the middle and the muscles on the side. Sometimes I will get a sharp, horrible pain that lasts only a few seconds. As I get older it gets worse but I am only 19 so I do not want this turning into a life long problem.. I feel like it could be related to stress if backpains are related to stress, I feel like it could also be a bra issue possibly, I am not sure though. Help please.