Why would there be mucus and blood in a child's stool?

My 3 month old daughter had this...the doctors freaked out and send me to the ER...which did no good. It was an allergy, in her case to the wheat I was eating, because I was breastfeedin. After all the test, and the attempts to run an IV (22 times of sticking her, and they never got it!!) That was all it was, just an allergy.