Baby keeps on getting sick..

My baby is 3 weeks old, I have been breast feeding her since birth. Most of the time after she finishes being fed she gets sick a little, but sometimes a LOT. Sometimes it looks like she got all the milk up and some days she gets sick after every feed. Is this normal? She is gaining weight though, last week she weight 4.170 kg and now she weights 4.320kg so I'm told not to worry about it but I still worry.. I feel like it's my fault that she's getting sick and I want to give up on breastfeeding..