My (now 7 month old) did the same - suggestions were teething (the suction makes the pains worse), reflux (she was arching her back and trying to sit up), or generally being too interested in other stuff to feed properly.
In the end I found that I had to either take her to a quiet room with no distractions, or turn my face away when she started fussing. Both of these worked - I think she was taking on enough milk to stop the immediate hunger then wanting to look around for the next diversion.
Other than that (providing there is no immediate medical concern, of course), just keep repeating "it's just a phase" which is what I seem to be doing all the time!
Good luck!