Yesterday, while at the doctor's office, we discovered that Katie had a tiny bit of blood in her dirty (stool) diaper. Her pediatrician immediately tested it, and learned that it definitely was blood. Last night, we had to get 3 stool samples to take back to be re-tested today, and just learned that Katie has a possible milk/protein allergy.
From what I have read, this is fairly common in newborns/infants, and odds are she will grow out of this by the time she is one. We have to discontinue all use of the regular Similac formula for supplementation, and switch over to a hypoallerginic formula called Alimentum. While this formula is a bit more on the expensive side, the good news is that she isn't using that much per day, and hopefully by next week will solely be on breast milk.
Probaby one of the biggest downfalls to this situation is that I will have to discontinue all use of dairy products until either a) the blood disappears or b) until I stop breastfeeding all together. Me without cheese, sour cream, cream cheese, milk? Wow. Once again, welcome to the world of motherhood, but you have to do what you have to do to keep Baby Katie as healthy as can be.
More details on this type of allergy can be found here.