Wow, that makes you a fortunate man...how you have done this?
It must be one of the many incredible glitches that fallout 4 has to offer. lol
Yeah, man save often. I say that myself all the time and sometimes i forget it completely. The game has a tense atmosphere sometimes, and a killed myself a couple of times because of this.
Anyway, i wish you luck, with your new game-through.
Also you should know, there is a new patch on the move, but first for the PC and then for the Consoles.
Who knows, if we are lucky we will get a patch next month before Christmas (?!?)
Have you done the Confidence Man quest?
I found that after doing part of this quest I could no longer die (the part where you take part in a fist-fight in a bar).