If you have persistent pain, please stop playing Osu and see a doctor
It is not worth sacrificing your physical health to play a circle clicking game
From your description ("i got my tablet yesterday", and assuming you don't have a preexisting injury), this sounds more like discomfort than RSI though, so you should probably try holding your pen in different grips until you find a comfortable one. Adjust your chair height, adjust your keyboard position, adjust your tablet position, always make your setup ergonomically comfortable before you play.
Never force yourself to play in a physically painful position, it will only get worse and it's easy to do permanent damage to your hand/wrist/elbow.
(Don't listen to the dude above and force yourself through pain! It is easy to react that way when you are young and think you're indestructible, but I promise you, ignoring pain will permanently fuck your body up. It is a very bad idea.)