I hear a lot about blood in the stool. Usually there are only flecks or streaks, and the baby is almost always happy and gaining weight well. Indeed, in the vast majority of cases, the babies are gaining more rapidly than the average. If this is the case, then it really is not something to worry about and I would ignore it.
It is worth seeing the baby's doctor just to rule out a fissure in the anus. Since many exclusively breastfed babies have explosive bowel movements, a fissure is not impossible.
If you are not reassured, yes, there may be something in the milk that has come from your diet that's causing allergic colitis.
In more than 99 percent of cases of "allergic colitis," stopping breastfeeding is inappropriate and any substitute for breast milk is still considerably inferior to breast milk. Furthermore, the "special" formulas designed to help do not always work.
Here is what one pediatric gastroenterologist emailed me about "milk colitis."
"I think that the GI (gastrointestinal) bleeding of babies is grossly overrated and doing less is better than doing just because. I like to scope these babies to prove they have allergic colitis. Once proven, one can easily follow progress with diet modification. Also keep in mind that it may take three to four weeks for severe colitis to resolve. So saying that diet restriction did not help after a week is inappropriate. As long as the baby grows, leave them alone and take time to do the challenges."
By this the paediatric gastroenterologist means that the seriousness of allergic colitis or milk colitis is "grossly overrated." In the majority of cases, there are only small spots or streaks of blood in the baby's bowel movements. There is no evidence of long-term harm to the baby, and thus the common practice of stopping the breastfeeding and putting the baby on "special" formula is not appropriate.