Pain on one side of the body, especially the arms, is very commonly caused by a problem in back and neck. When muscles and joints in the upper back and neck are not working properly, the resulting dysfunction can cause pain and weakness in the arm. The specific location of the dysfunction depends on where in the back the problem comes from.
Another common cause of forearm pain and weakness is tennis elbow if it is on the outside of the elbow to the hands. Here is our youtube channel, I will be doing a video on forearm pain. https://www.youtube.com/channel/UCMWQtawahy7vj6SoySDFw-g
Lots of things cause this pain, such as heart problems, lung problems, diabetic high blood pressure condition, drug side effect problems, fatigue, sprain, strain, and others. Please see a chiropractor or any primary doctor for a consultation, and as for me, I would direct you immediately to consult a cardiologist surgeon for an EKG, ultrasound, and catheterization, which will probably solve your problem.