Hey this is happened to me a few times and I have no idea how to fix it, other than loading my previous game. Sometimes the light will be so bright that I can't even see anything on the pip-boy, just a very bright green light.
The cause may be from going to your pip-boy right after launching a mini-nuke or something that causes a bright explosion, and for some reason occasionally a robobrain cripples my head and I look at my pip boy it's bright.
Yeah, My pip-boy went like that near Dukov's Place I just held the B button(the pip-boy light for PC + PS3 users).
This is a gamebug, simply hold the pipboy light button and toggle it a couple of times.