Because of your meds?
The ones that make you not go ax-crazy?
...which made me think
Commandos - Special military units that probably kills a bunch of people
Postal worker - Not very special people who hands out mail
Going commando - To wear no underwear
Going postal - To flip out and kill a bunch of people
That makes no sense.
Going Postal is attributed to a spate of US Postal workers in the 80s and 90s going nuts and killing people.
Apparently, according to the link above, there's been a study to show they're less likely than most, but there were a few all around the same time, so the phrase stuck.
As for the other, Wikipedia has some interesting theories.
The one I find most likely is it being a modernisation of "Going Regimental", mixed in with the Vietnam War commandos