While I don't pretend to be a plumber, I have a fair amount of experience troubleshooting a vast array of common household ailments. The problem is probably in your shower valve. If you have a single handle cartridge type (Moen, Delta) the cartridges can fill with minerals and cause distribution problems. The easy fix is to replace the cartridge, available at Home Depot, Lowes, or other. The part is about $10, it will only take ten minutes and the valve will work like new. Good Luck.