What Causes Inability To Move/fold Middle Finger After Delivering A Baby?

HI, I am 34 year old mother of three children. I am experiencing an inability to move or fold my middle finger on the left hand. This happend right after my third delivery. Initially it was only on the finger however after some six months time it getting worse and the pain reached to the wrist. Meanwhile the whole left hand has got little less stamina than the right hand.

Could you please give me your valuable advise how to proceed with this case and what could be the reason and how to cure this problem.