Hi I haven't had time to read every post so I hope I'm not repeating anyone else but just wanted to comment.
My daughter was breastfed and by 6 weeks losing weight rapidly, constipated and a very unhappy baby. Eventually hospitalized, I was near breakdown, and drs, midwives and healthvisitors were useless. She was eventually diagnosed with milk allergy, I went dairy free and tried to continue feeding, but she was still not improving, so they diagnosed soya allergy too. Tried cutting that out too, really difficult because its in so much food. We had all the reflex meds and lactase stuff but nothing worked, it actually seemed to make it worse.
At this point she was still constipated and needed suppositories daily. Doctors kept saying it couldn't be caused by milk allergy as that would give her to opposite problem. She was given Nutramigen formula and the transformation was astounding. She started pooing by herself, started smiling and turned into a happy, jolly baby. She gained weight, no wind. Excema cleared up completely. I've since learnt that milk allergy can cause both constipation or the runs.
Insist your GP sends you to see a paed and dietician. Try giving up soya as its proteins are the same size as milk proteins and can confuse the body. I felt very guilty for a long time for stopping breastfeeding, but with hindsight it was the best thing I did for her, as my milk was making her ill and hurt. They now think she was allergic to my milk, which is very unusual.
Shes now just intolerant to milk, and can tolerate small amounts for treats etc.
Hope this helps, keep your chin up and follow your instincts.