It could be a TMJ or some neck muscle issue. Nothing serious, just something to see the chiropractor about. I get what you're experiencing sometimes too.
For me, the "numbness" feels more like that sensation you get on half of your face right after waking up and taking your face off of the pillow, except the feeling would stay for most of the day on most days. That was more anxiety related for me I think though. Anyway, tons of harmless yet annoying things can cause what you're experiencing and they're all more common than tumors, MS, ALS, or whatever else you're worried about. If you're truly worried go get an MRI for some piece of mind. I got my results back as normal yesterday and I already feel so much better and the numbness has seemed to go away, and I bet yours will too once you find out that you're okay :)