Sustained twitching

I have these episodes where a muscle in my arm twitches repeatedly for, say, 20 minutes—erratically, multiple times a second or with longer pauses between. It doesn't actually cause my arm to move; I can feel it and see it under my skin. It's sometimes but not always in the same place. It's pretty infrequent and tends to be after I've been leaning on my arm for a while.
Web search suggests that the best case is a direct result of lactic acid buildup and worst case is ALS. Is it normal?