The recommendation for Brain Growth Work is once or twice daily, or every other day (from the description below). 
Overusing electrical stimulation can result in issues like you’ve described, amongst others.
It is best to follow the recommended guidelines for this field, and for all fields inducing electrical stimulation like this or Permanent Cognitive Enhancement or Superhuman Genius, better to begin slowly at first, then observe your response after some time has passed.
(start with less listening time rather than potentially too much)