Not sure what to do about my babies formula?! At a loss......?

My baby is 5 weeks old and I fed the first 3 weeks but due to supply I had to go to formula. I tried everything to produce more. Anyways, we got Good Start Gentle Plus due to WIC covers it. She spit up horribly. Projectile most of the time not to mention the constipation. So the doctor suggested Enfamil AR over the phone, we have tried that for about 3 days now and she hasn't pooped. Spit up was good first day then since then it has gotten worse and even projectile last night. So due to the doctors office being closed I wanted opinions on this. I'm thinking maybe a milk allergy now but not sure! Suggestions anyone? Please. At a loss here cuz I don't know what to do.