Maybe peripheral neuropathy? It seems it can show up a very long time after chemotherapy. Damage to the nerves that enervate the muscles can cause weakness. The pain may be from the nerve.
https://www.cancer.gov/news-events/cancer-currents-blog/2017/peripheral-neuropathy-chemotherapy-breast-cancer
But there may be other reasons you’re having those symptoms. Overuse can sometimes make the tendons or fascia hurt and limit motion. Other things to consider: a pinched nerve, carpal tunnel syndrome. Or, peripheral neuropathy from other causes like metabolic or disease (although these would probably cause problems in both arms not just one).

When I have issues my survivorship provider always wants me to check with my regular PCP or GP first to rule out everyday health issues before going to the cancer center. You may want to check with your doctor to find out who they want you to go see. There’s a whole group of people on here discussing peripheral neuropathy- they might have something helpful.