I'll mention this because I havent' seen this in previous posts-- I have two children, both of whom started to have trouble at the breast at 4 months. The first time my daughter was diagnosed wtih reflux and she was prescribed Zantac which did exactly nothing to stop the arching and crying at feeding time. She also had mucous in her poop on and off. It passed with time and I never figured it out.
Second child three years later and same pattern started at the same time. My pediatrician friend mentioned a website to me and said two things that made so much sense I knew she was right: I had a forceful letdown of milk and the arching and crying was a protest at being drowned in milk! As a result of a difficult feeding, babies would often only drink foremilk, which is lower in fat and higher in sugar and can cause the mucous in the stool. There's no cure for this situation, just patience and some possible modifications to breastfeeding. Knowing what the problem was helped me empathize with my baby and also gently coach them through. The problem lasted for a few months, then they grew big enough to cope with the intense letdown.
As a follow up, my daughter does have environmental allergies (dust mites, cats, dogs) but my son so far has no allergies. I think there was no relationship to the mucous in their stool and allergies. My daughter didn't show allergic symptoms until she was two.
And we also have Kaiser. The pediatrician will be more inclined to prescribe Zantac and diagnose for GERD. Try getting an appointment with a lactation consultant. Kaiser employs many of them and they're great. The pediatricians seems to have little or poor knowledge about breastfeeding.
The website is: http://kellymom.com/