Baby Having Diarrhoea. Having Low Blood Count. Eats Well. Chances Of Having Leukemia?

My pediatrician told me to put my daughter on a vitamin after doing the finger poke test at her office. She is 15 months old. She has had diarrhea for about four days before we went (not full blown diarrhea the entire time, just different consistency) could having diarrhea have caused her to have a low count. Her count was 8 and some change. My concern is all of the potential causes you read about, the scariest being leukemia just want to make sure this is a fairly common thing. My daughter eats well, we have fruit for lunch everyday, and except for on pizza nights we have meat and veggies, and she is my best eater