My right thumb pad twitches NON STOP. It's nice to see other people with twitching thumb problems as well :)
mine has spouts where every time I flex it or use it in the slightest, it spasms for a good 1 or 2 minutes... its completely benign.. just do to some nerve pressure from playing guitar and typing ( I work from home )
Also, if you had anything serious, your thumb would not work properly. If you can still write, pick something up, twiddle your thumbs, your nerves are in perfect working condition :)
also please be aware that there is a Percieved weakness and actual weakness.. if your thumb FEELS weak ... like its not preforming at 100%... it's probably because the muscle is exhausted from twitching.. my thumb does this as well.. the next day its fine..
most likely you have a pinched nerve in your neck or back (if you have any pain or discomfort thats a dead giveaway) or somewhere in your arm.. mine is actually in my thumb itself, if I stimulate it to much it pinches and spasms out.. it can last for a day, days or even weeks.
Don't worry, your FINE :)