Is this MSPI?

DD has had reflux since birth. We put her on Alimentum per the doctor’s advice only due to the gas she was having and tummy pains while drinking regular Enfamil/bm. Later we switched to Nutram as it’s almost the same thing and less money. They never said she had an MSPI. They just said they thought it’d be good to use while they try and get her reflux under control. So she’s now almost 11 months old and has been doing super. She’s on 15 mg of Prevacid. I think the right dose of Prevacid did more good than anything else really. So we asked the GI if we could switch her to a regular formula. He said that was totally fine by him. He asked that we go w/ a lactose free formula just in case she got really gassy.
So we switched her to Similac Sensitive. I did a 1 1/2 week transition. No issues at all. She’s now been on it for three weeks straight. This week she’s been constipated beyond belief. Even Miralax isn’t doing much. On top of that she had a little bit of a rash on her inner thighs-looks like little pink pimples and this morning she spit up all over herself. She has silent reflux so she maybe spit up like 3 times in her whole life. So this isn’t the norm. So I don’t know if the constipation is making it worse and she’s spitting up from that? She wasn’t fussy when she spit up. She’s been in a really good mood. Could the rash and the spit up be related? I mean is that the sign of MSPI? Or could that just be heat rash?
I’ll do whatever I have to for my daughter but it’ll sure be sad if she can’t tolerate regular formula because that means she’ll probably not tolerate milk either and then she’d have to be on Nutram forever. 🙁 We see her pedi later today but I wanted your opinions for now… Thanks!