Omnicef and medical attention

My 4 yr old son was given Omnicef for pneumonia. After a few days, i saw what i thought was blood in the stool....his stools have all been soft, but not diarrhea and also full of mucous. my son dances around crying to have a bowel movement and it breaks my heart. i called his ped who told me too that it was the dye in the med, but hello, the med is white. I have definately seen blood in his stool...bright red. i looked at the website for Omnicef and it says if you see blood in the stool or mucous to stop using it. He's almost done with it so i am going to stop it. My daughter is 2 and is on it as well and so far, no blood in her stool. this stuff is scary. Does anyone know if the blood goes away after you stop this or do you need to seek medical attention?