You can get it from some of the muscles in your forearm being too tight, which can happen from inactivity. Notably, it was one of the things my physio gave me stretches to avoid after I broke my elbow and was stuck in a sling for a couple weeks, which left me with a very stiff forearm. In that state, I found that the repeated thumb motions of playing controller-based games gave me some tennis elbow, so maybe yours has been caused similarly.
You can stretch out your forearm by holding your arm out straight, grabbing your outstretched hand with your other hand, and gently twisting the arm until you feel a stretch in your upper forearm (both directions can offer stretches). Don't go too hard, since you don't want to hurt yourself, but it can help loosen things up.
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