My DS thrived yet develope really mucousy diapers around that age. Actually, he continued to thrive despite a deteriating digestive situation.
I tried elimiating milk and soy (+peanut butter as it is closely related to soy) from my diet, which was very difficult...especially in December. I was very strict and would not even eat things with the warning that they were manufactored in a plant containing other milk items. After 2 weeks of no change, I also elimated wheat.
Determining your child's specific allergen/s can be frustrating. We ended up taking him off BM so his little colon could heal. And unfortunately for all of us, he still can tolerate EBM free of milk, soy and wheat. I wish I knew a complete list of his allergens but the discovery process was too difficult on him. BUT...I now have an extremely happy baby...as long as I don't mess with his diet.
I would support talking to your doctor as s/he can do a quick test to make sure he is not bleeding from his colon.