Some babies get very impatient if mom has a slow let-down. More growth spurt information in this link. Walking around while doing this might distract her long enough to get a good burp. Has she started solids or is she trying a new food? On a similar note, an occasional baby will just want to suck at the end of a nursing session and the flow of milk with let-down frustrates her. If baby seems to be pulling off the breast at any distraction real or imaginary , then see The Distractible Baby.