Spencer's symptoms when his allergy were picked up was ezcema and really bad constipation, he also did that back arching and screaming at feeds and vomiting constantly, even hours after a feed. I really hope you get it sorted and definitely push to see a paed. Plunket told me to put him on goats milk formula and then try soy, totally wrong advice. And the gp I saw when Spencer had been up all night screaming cos he couldn't poo told us to "just keep trying different formulas"
Lucky that wasn't my normal gp otherwise I would have changed surgeries. It really annoys me how gp's and plunket are so unaware and uniformed about allergies when they are so common.
If you want to do something about the constipation in the meantime you can give lactulose. Spencer just use to have 5ml a day to start with and then adjust the amount up or down depending on consistency of poo. It shouldn't be a long term solution but might help things whilst you wait to see a paed.
Spencer got so badly constipated we (well me actually) had to give him an enema
was not a very pleasant experience for me or him.