3 week old baby suddenly fussy during feedings?

My 3 week old daughter is breastfed. She refuses bottles and pacifiers and formula, anything that isnt breast. She has been wonderful at breastfeeding and just the past two days, she has been fussy during feedings. She will drink for a minute or two, and as soon as the milk lets down and starts to flow at its strongest, she clamps down on my nipple, gets fussy and wriggles around. She will let go of my nipple and then cry, and reattach herself moments later and repeat the whole process for 2 hours. This happens mostly in the evenings. Any ideas as to what is going on?