From the way you've described this forearm pain, i believe i have had literally the exact same issue. Mainly would get it with heavy barbell curls at first, and as it got worse and worse over a couple months i started experiencing it when working shoulders and chest too. To me, it felt like tendinitis. I never had it diagnosed but it was similar to the pain i experienced when diagnosed with elbow tendinitis, except this was in my forearm like yours, and it hurt most when i would release my grip, just as you stated. Every time ive had this pain flare up, the only solution was rest, and after 3 weeks or so it would be completely gone. I found that the main movement that was causing this was heavy barbell curls. That was the only thing i had to change. Once i switched to DB curls, the pain began to subside over a few weeks, even though i did not take time off and did not stop benching and doing shoulder press. Figure out what seems to be the main antagonistic movement and either change it or stop it completely. Also, tiger balm worked wonders for me as far as alleviating pain after workouts.