About your hand, it's probably because you're using your wrist too much when you're drawing.
Lots of artist might suffer tendinites there because of constant use of the wrists. To remedy that (or diminish) you need to use your shoulder more.
Here's more info about how to hold the pencil (digital pen):
PS: you don't need to adjust your grip to a Hand-over grip; keep using your grip but just adapt your wrist to shoulder more when drawing.