Best formula

I have a preemie baby and she’s on neosure but I notice ever since she came home we’ve started to use the powder and she’s constipated with a lot gas and her face has a rash. Took her to the dr and she said the rash is normal, looks like little pimples. Now I checked the back of her head and found this 😰😭 I have no idea what that is and I’m gonna take her to see her dr again tomorrow. I wanna switch formulas, which one do you guys recommend best ?