Baby in pain while nursing?

Help - 3rd baby, everything was going smoothly nursing until the last few days, 5 weeks old, I use a nipple shield, and suddenly it’s like my milk is acid. She’s screaming and kicking or going stiff. My supply is good so I know it’s available. Is this terrible gas? She nurses fine at night but it’s now rejecting/screaming during all daytime feeds. I’ve resorted to pumping and bottle feeding for my sanity and so she actually consumes something the last day or so but I feel like that’s not helping the goal of breastfeeding long term if she likes the bottle better? Starting probiotics. Any ideas/Has anyone else gone through this?