That's a perfectly normal reaction. My cat does that too. And your cat's tongue is supposed to be scratchy. It has little "rasps", as I like to call them, that serve several purposes.
Here's a video of another cat licking itself while being scratched: https://www.youtube.com/watch?v=40e8QCiMLBY
And here's a picture of a cat's tongue: http://bzfd.it/1GzqgFo (link was long)
Some cats lick themselves wildly and just go crazy, but some barely react. One way or another it's perfectly normal.
Answered Feb 27, 2015
Edited Feb 27, 2015