I'm having a similar problem--except 1-week later, the itch has turned into a bad rash. The only change in my life (I'm not sexually active) has been "extreme juicing" due stomach acid problems I developed 3-years ago. I began self treating with antibiotics I had "in stock" and using "Desitin" high in Vitamin E--normally used for baby diaper rash--! I have upcoming appointment with Gyn.
Hair shampoo has harsh detergents in it. I mean, have you ever gotten it in your eye? Imaging what that could do to your delicate membranes down below. Given the circumstances, I think there's a very good chance you had a reaction to the shampoo and it will be getting better by this evening. If not, try some of the anti-itch cream from the pharmacy. Good luck.