Could be something more simple like inflammation of the tendon or muscle. Ive had the same thing for years. All my weights dropped considerably. Doc put me on 15mg of Melixocam ( much more potent than ibuprofen without the pain meds) and after 3 years the problem was fixed in 3 days. every few months i have to take it again and its fixed...
I may not be right but may be worth it to look into.