Bony back problem?

Well I'm at a pretty normal weight (I'm 5'4 and 108 lbs) BUT I have a pretty bony back. I don't know why. My spinal cord and shoulder blades stick out too much and it doesn't look very nice. I don't know what to do because I have fat elsewhere. My back also hurts when I sit against a hard chair for too long. I don't think I can fix this any other way than by gaining more weight, but I don't want anymore weight on my lower body. So what do you think I should do...accept my bony back or put on the pounds? Thanks.