Do you use a lot of pressure when you're drawing?
Try ease that!
How's your posture when you're drawing? Do you slouch? Are your shoulders tense?
Things like that can also be a problem for the pain.
I've got tendinitis and it can be a bit of a pain sometimes, stretching and pausing my work has done a great deal for me!
You can look up different exercises for that on youtube for an example.
However, if the pain persists and you feel a tickling/numb pain in your pinkie finger, that could be CTS which you might need an operation for.
If it's "only" tendinitis, you're in luck as it's not as terrible as CTS can be.
And a tip, if your wrist is in pain, it's in the elbow. If your elbow's in pain, it's your shoulder. If your shoulder is in pain, it's your neck.
That can be a bit tricky to know, but for me it usually helps by massaging my elbow muscle (both upper and lower) and the next day the pain is a bit less.