You might be tensing way too much - which is common. To see if this is the case, first do a tensed hard fist with your right hand and notice how tense and engaged the muscles in the middle of your lower arm (forearm) are - that should be right around the area you mention. Now try a normal arm/hand position and pluck the strings - after a few do you get the same area tense and engaged? If so this could be why the ache is there.
However, simply resting more is not going to correct this - you should not be tensing that much at all when practicing your plucking. To try to correct, you could try being conscious of the tension in the forearm by playing each stroke and letting 2-3 seconds pass before you play another. In that time, consciously relax your forearm. Do another stroke, and relax again. Just do t his for a minute or two, then try playing at your normal pace. After some 5-10 min try the pluck and conscious-relax again for a minute. See if that makes a difference.
1982 Anselmo Solar Gonzalez