Sort of a silly question, but I have noticed lately that I have been getting some really good warmup sessions leading to very smooth playing. Then it could even be the next day or hours later that I can't warm up anywhere nearly to that same level. It's as if I forget how to hold the pick properly or something odd like that...

Very frustrating
Yea, I definitely have those days where I feel like I'm at a lower level than before. Usually it's my strumming and accurate string picking that go up and down. For me, I think, a lot of it is mental. If I'm feeling really good about where I'm at and have a good, varied practice routine with no distractions I notice more improvement than when I'm just thinking about how far I am from my goals.

Although I've noticed this happen with almost everything that I've put a lot of time into learning.