It's not likely to be herpes without any sores, but stop taking the Valtrex and see what happens. Herpes typically doesn't go from the genitals to the mouth as easily as it does from the mouth to the genitals.
When you say you took Valtrex before you had sex, do you mean you take it daily? In order for Valtrex to work to reduce transmission, you have to take it daily for at least 5 days prior to having sex.