HELP. :( switched formulas

Ask here while I'm waiting to go Dr's tomorrow!!!

I switched my daughters brand of formula the other day because of constipation:( from Enfamil A+ ready to feed.. To Similac Ready to feed. She pooped as soon as I gave her the Similac. she has pooped since? but it seems like she needs too. I tried a really warm bath (family member suggested it). Didnt help :( She is 6 weeks! at her last check up the Dr we seen suggested we switch up the formula.....