Both my kids were CMPI babies. DD (my first) was much worse than DS, which I think was because she had formula from birth and therefore a more 'direct' exposure. I also breastfed, but due to a complication after her birth I wasn't able to feed her on the first night and my supply never properly established. As far as symptoms, she had frequent runny poo that would range in colour from bright green to almost black and it was often frothy and/or full of mucous. She also had reflux, would scream all day everyday, was very slow to gain weight (failure to thrive), and was fussy at the breast but also wanted to comfort suck. It was a nightmare. After basically starving myself to avoid all traces of dairy or soy (suspected soy protein intolerance as well) I gave up at 13wks and put her on Neocate. Total miracle cure in her case. She's 4yo now and can have any dairy without a problem, which amazes me given how sensitive she was as a baby.
DS was exclusively breastfed until 17wks and then I slowly started introducing AllerPro. I stopped BF completely at 5 months. His main symptom was the mucousy poo and a stool sample showed traces of blood, suggesting irritation. He's 16mo now and has cheese & yoghurt and meals with dairy used in cooking. He's fine provided we don't give him too much in one day.
If you can, grab a good sized stool sample in a sterile collection jar to take to the GP. It's pretty much the only diagnostic test they can do - CMPI is usually diagnosed by a 'constellation of symptoms' and blood in the poo is a pretty important part of that.