Why are veins randomly popping on my fingers?

I have veins popping ramdomly on my fingers that are painful. For no apperant reason I will have pain in one of my fingers and then I will look at the finger and there is a vein ready to pop. When it pops it leaves that part of the finger black and blue. It is a very painful process and I would like to know if someone knows why this happends. I take no medications nor have any health issues except for this!