My LO did the same thing for about a week - it was distressing. She'd fuss and fuss while drinking and seem to want more and latch on and then fuss more. It was tough when we were out visiting and it would happen, I didn't know what to say to those we were visiting to explain her behaviour...but...as with many things, it passed...for the most part. On occasion she still does it and I still have to go through whether she is being distracted, whether she has to burp or whether to switch breasts - the last one seems to be the answer most of the time. For that reason I'm led to think it's a growth spurt and they are drinking faster and emptying out the breast before the breast can replenish and so when they go on the other breast they are often much more content because they are getting some, and getting it faster.