I have almost the same problem. I`ve been chanting Mahamantra 1 round during last month and everything was good, i enjoyed it. But one day ago i decided to increase my chanting for 2 rounds and after i started the second round I felt emptyness and even fear and also strong desire to stop chanting. Then all day long I did not want to chant or hear Mahamantra. This morning as usually I sat on mat and prepared to meditation but I could just chant narasimha pranama mantra and Sri Krishna Caytanya...
May be somebody knows if this pocess is natural during spiritual growth or it is something else?