I assume the problem wasn't there before the rough-in, right? If
that's true, then the pipes you installed are somehow crossed. To
eliminate your hope that it's the washing machine, shut the hoses off
I think you either ran a hot water line to a cold outlet, or you
somehow looped them together.
Of course, it could be something freaky, like a recirculating line,
but these aren't usually put in home installations. If you do a little
more detective work, please write back with the info.