What Led To A Tiny, Painful Blister On My Finger?

hi doctor,yesterday, i came home from work and i saw that i had what looks like a tiny water drop on the inside of my little finger, It hurt, especially when i touch it or when it rubs against my other fingers. I work in a hair salon as a junior so i am constantly washing poeple's hair, and always in contact with hair products.I wonder if i could have caught something from someone's head or an allergy to a shampoo??