Hi there ~
I understand your concerns. Weakness in hands and fingers with soreness in underarm seems to be signs of a pinched nerve. If you put pressure on a nerve root
or a nerve bundle for a long enough time you might feel that the area is tingly or numb or otherwise does not feel right. Movements may be affected. This probably was what happened. However if it has been happening more than required I would advice you to consult a neurologist
I hope this helps. Take care of yourself and have a lovely day!