One word: ugly. For Central Europe residents: just think of Kája Gott (Czech pop singer) before you curse your child with this abomination of a name. Yes, and Anonymous is right, Kája is not an official name, just a pet name.
Kaja is also a pet name of Kajetana. My Czech mother is called Kajetana and she passed her name onto myself, I am Slovakian. It is a very uncommon name and I believe it is derived from the Italian name Gaetano. He was made a saint and our name day is therefore celebrated on 10th August according to the Catholic Calendar. It is pronounced to rhyme with "Hiya".