Pain in finger joint?

I have had this strange pain in my middle finger joint. It is not a constant ache, however, when I pick up something hard-like a suitcase, the hard surface of the handle comes into contact with the joint of my finger and I have a sharp pain. After that, it is pretty much sensitive for a few days. It kinda feels like a nerve is touched. It bends fine, is slightly sore on bending it but really painful when it comes into contact with hard objects e.g. poles on busses, arm of a chair, etc.. It drives me insane as I have to try and avoid these things as the pain is like a stab. Any ideas?