Ever time I take a shower, as soon as I start using shampoo my noise starts itching. I never had this problem when I was younger. I tried a number of different brands and it still happens. I am not allergic to anything (else) and neither are my close relatives.
Did you try something hypoallergenic? Maybe see what happens with the shampoo open not in the shower, it could even be a reaction to something else scented, soap etc in the bathroom that took a few minutes to have an effect.