Almost 3 week old has bad reflux with formula but not with breast milk

Has anyone had experience with this? She is fine when having breast milk but when given formula (we have Gerber good start Gentle through WIC) she projectile vomits almost every bottle we give her. I have very flat nipples so I use a guard but she’s now gotten to the point where she won’t latch on to the guard and when she does she only eats a little bit and stops so my supply is running out fast. So she’s going to be strictly formula fed by a month old I’m thinking. WIC changed our formula to Gerber Soothe. Has anyone had luck with this? She did really well on a sample can of Enfamil Gentlease and Similac for sensitive tummies but WIC won’t give us those without a prescription. I can’t call her doctor till Monday so I need to find something to last her the next few days. She was up all night last night with a tummy ache. She doesn’t have a fever or any other negative symptoms so I don’t think she’s sick