Are you in the process of replacing the shower valve in the bathroom? Is the cartridge installed? Two things that come to mind (the pros may have other thoughts!) are: no cartridge in a rough-in valve or a worn out cartridge. Sounds like you have a cross-over. When you shut off the water, if there's any crud in the pipes, it can get dislodged by the inrush of water. This could mess with things, too. But, since it happend immediately after work, look at that work to identify the source of the problem.