What Causes Bleeding From The Scalp While Combing Hair?

Hi i am 14 and my scalp was really itchy so i went to go check if i had headlice but when i combed my hair all this dandruff appeared after the fourth comb i found some blood appearing. What is happening? should i just go get some dandruff shampoo? also my hair was really oily at the time, because i hadnt washed it in a while