Hi! I know people with problems with that too. You might find this useful: Book: Conquering carpal tunnel syndrome and other repetitive strain injuries: a self-care program (1996) by Sharon Butler (You might find page 36 interesting) and Brandon Dayton YouTube Channel: Managing Hand Pain for Artists.

You gotta watch out for that! I'm prone to hand/wrist/shoulder injuries, but I've never felt "stiffness" in the forearm. You probably need to take a day or two break, and make sure you take frequent breaks during drawing, like every half an hour to an hour. Set an alarm for it. Where in the forearm are you feeling? What's the posture when you draw, like do you raise your arm to draw on the easel, or flat on your desk? Do you also use a lot of mouse or type a lot in addition to drawing? Sometimes it's the mouse or keyboard that breaks the camel's back. Look up "forearm pain physical therapy" and find exercises that you feel right for your condition.