Just another reason to store in glass. I've found that the soft squeezable plastic bottles "breathe". You can actually smell flavorings in them. Just threw out a bunch b/c of that, they were a year old though. I figured if air is getting out, air is getting in and other stuff.
If purchasing nic liq to diy, definitely switch to glass, even if it is in the fridge. If you don't plan to use it all up soon, it will turn color fast in plastic.