This is a fantastic question. I have often experienced that feeling. I doubt that there is a succinct word or phrase to describe it. I suggest you coin your own word and use it all over the place until it finds its way into a dictionary.
In general, when you say or look at a word too many times /too long the word loses the affiliation it has with its meaning. It starts to be nothing more than a group of noises coming out of your voice-box or a collection of alphabets arranged on a page. I had someone once describe the feeling to me as word-dissolutionword-dissolution because to him the word simply dissolved. The brain has already understood and processed the word. Now it knows everything there is to know about the word, and has moved on.