I have been having bad pain in my left thumb for a month now?

44yr old female. Take lisnprl for hbp and effexor for bipolar. I have been having bad pain in my left thumb for a month now. I have no problem moving it back and forth but to bend it at the joint hurts to the point that I have to use my other hand to straighten it. I did not have an injury to it beforehand. What is going on?