What Causes Numbness And Tingling In Fingers And Feet?

I am 47 years old and have had numbness and tingling in my fingers and feet for over a year now. They go white then purple and numb, I work as a cashier and have trouble counting change sometimes because I can not feel the money.sometimes it is the slightest temperature change and it can be warm out sometimes it is one or two fingers or toes or all of them.I am on Hytrin 5mg twice a day and still have this problem is there a way for it to go away or minimize the effects?