I just caught up with this and wanted to share our experience.
When we brought Dempsey home until he was around 3 1/2 - 4 months, we had problems. Loose stool that would, on certain days, become mucousy or bloody diarreah. He would be come VERY lethargic, and occassionally throw up. This happened 4 times before we found the problem!! It was very scary each time, especially the one time we had to leave him there over night! They told me it could be parvo, etc, everything that could make you terrifed for your baby! I went to three different vets and finally we narrowed it down to a food allergy. He ate Science diet for about 3 days and started feeling much better, so I tried to find a food with little ingredients. (He was on 2 1/2 bags of Wellness before he reacted to it). Wellness Simple Solutions, he never did well on - I think (don't know for sure) that it was the oatmeal. Now he is eating California Natural Lamb and Rice and is doing great! He gets the occasional loose stool from stealing little treats he shouldn't, but other than that, nothing! He eats Lamb and Rice training treats, and I am picking up a bag of Naturals Balance today to see if that will be a good rotation food. So he doesn't do well with Oatmeal, and I think he is allergic to wheat. I figured this out through elimination. It was very frustrating.
If this is the case with your baby, then I truly hope it helped. I suggest try the California Natural Lamb and Rice. The reason for Dempsey's nasty diarreah and lethargy was because his intestines would become inflamed. The pain he wass in made ihim lethargic and depressed. I really hope you get this figured out! <<<<Healing vibes>>>> Good luck!