Rash after formula? 7week old

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From the start I was exclusively pumping.

My supply is almost dried up so we decided to go to formula.

We went with enfamil.

So we started with formula and throughout the day we just went back and forth between breast milk and formula...

By the end of the day he had a full body rash.

We stopped immediately and by the next morning it was pretty much gone.

What do we do? He has a appointment in 2 weeks but I don’t think I’ll make enough milk to last that long.

We want to start formula.

*no, I don’t want to build my supply (unless I absolutely need to).

We talked about going with formula for a little bit now.