Could be colic. Gripe water works pretty good. Along with moving their legs in a bicycle motion and laying them on their tummy and rubbing their back and then tummy.

It could be gas as well.

Ohhhh yeah. My twins went through that as well around 3 weeks. It was a grow spurt,

could be a growth spurt too!! they have ones typically around 3 weeks old. I remember when my daughter had hers. she was either fussing and crying, eating, and at one point sleeping a lot during her growth spurt. it's pretty normal if that's what's going on.

Sounds like baby is colicy!! Try burping very well and feeding in a more up right position.