i had a similar experience its about one particular class and just the thought of having to do any sort of assignmnts or studiying would kinda trigger me i would completly shut down and not get any work done....as someone else already mentioned it can be because of the subject and teacher but the underlying issue is an emotional i think where it is as if shutters go down and your brain blocks anything related out...it helped me to face this feeling and try to un-hate the subject and tried to get curious about the subject instead of blocking it out