Maybe you are allergetic to which the glasses frame is made of. Check the nose pad to see if there is any dirt on them. I had the same problem before, and I cleaned mine, and then the problem went away.
Maybe your skin is allergic to the material that is made of the plastic nose pads. Or the dirt on the nose pads cause your nose itching. You should clean your nose pads to see whether your problem will go away. If your nose is still itching after you clean the nose pads, you should go to see your doctor to have a check and see whether you are allergic to the material of the nose pads.