You'd have to see a dermatologist for that, I think. It might be that your skin is sensitive to something in the shampoo. A lot of people use sulphate free shampoos because they're less harsh on the scalp.

That said, there are people who forego shampoo entirely and it works for them. You seem to be one of them. Honestly I rather envy that! My scalp goes itchy and sore if I take more than three days between showers.