Baby fussing at breast

Anyone able to help? My baby is 4 weeks old. she always nursed so well aside from a shallow latch which I use a nipple shield for and she's done great with them. But the past few days, she will eat for a minute or two then cry and pull away. when I get her calmed down, we do the same thing over again. I'm not sure what to do as she's never done this before. Any experienced mamas able to give advice? I'm feeding her pumped milk in a slow flow bottle right now since she takes it better but I don't want to move to exclusively expressed feeding unless I have to.