Why My Head Roots Hurt When I Move My Hair.

Why my head roots hurt when I move my hair. It only happens when I blow dry it, and my hair is clean, but I don't really understand why it happens, I have really strong hair, it is naturally curly, but I like it more when its straight, I've always done it (blow dry it), but lately I've tried to have curly also, like have the month curls and the other half straight, and since I started doing that, my roots started hurting when I have it straight and I move it arround.