This happens to my shower head as well - every couple of weeks, we soak it in limescale remover or vinegar....
The limescale is probably also affecting the inside of your shower. Every couple of months, the temperature on our shower goes mad alternating between hot and cold every few seconds! There is a small filter inside the shower that normally needs to be soaked in something when this happens - because it is so clogged with limescale as well!