biting toes and fingers... acting out...is it a disorder?

My daughter is 5 years old. Sometimes she acts out and she likes to repeat herself alot. Its like i dont hear her when she talks to me... And she gets emotional about alot of things. She always repeats the same question like can i have candy over and over and over throughout the day. If i say no because I am making dinner she gets mad and upset. Also she chews on the tips of her fingers and toes. It looks really gross, sad to say. I dont know how to handle that. She has never done that before. She will chew the skin off and peel it. Also she wants to learn writing and reading. and as many times i have sat over the last year with her and her preschool has too, she just cant get the hang of it. She mixes up her letters with her numbers and she cant remember them if I ask her what it is. I dont know if its like dislexic or somethin another. I am trying to figure out how i can find out. Do i have to go to a special doctor for tests, or she just acting out for attention??? Help me out anybody. I have done everything i can possibly think of.