Baby may be going through a growth spurt. During a growth spurt, breastfed babies tends to nurse more than usual and are often fussier.
The increase demand of nursing is temporary. It usually lasts for 2-3 days. You can continue to nurse baby, not necessary to give expressed milk as growth spurt do help mummies in increasing their supply as well.
For more information, you can read them here:
http://kellymom.com/hot-topics/growth-spurts/
Read more