Hahah. As soon as I saw the topic title, I immediately wondered if you have traveled to the glow recently
Y'see, When you enter the glow, you suddenly start absorbing lethal amounts of radiation, you can pop some Rad-X when you arrive, but your current radiation level will remain. When you travel the world map, days go by in seconds, and thus, any immediate risks are amplified, and pretty much result in instant death. The Glow is usually the occurence of such a thing. Usually as you're leaving the Glow, your Rad-X is still working. If you try to leave after entering without taking Rad-X, you'll hit a lethal does straight away and perish. The same can happen if you absorb lethal amounts, Rad-X yourself, then die on your travels as your radiation count clocks over the fatal limit.
I suggest you check your radiation resistances on your character sheet, and take some Rad-X accordingly. If you're at full resistance, take some RadAway and then leave. If you have Rad-X but no RadAway, pop the Rad-X and search the underground facility for some RadAway (you'll need a rope to get down there) if you have alot of RadAway and no Rad-X, take it all, leave, and pray you don't die. If you have neither, then you're screwed. If this is the case, I can only hope you have an earlier save to recover from.