Herpes. It's a virus. Many people are carriers and sexual transmission can pass them along even if you are monogamous....You both might have been exposed in prior relationships and it is manifesting itself now.
Sexual/oral route. Herpes virus is transmitted with contact: skin to skin, mouth to mouth, mouth to genitals, genital to genitals. It can lay dormant in sensory nerves for a long time before becoming active. Think carefully and talk to a physician, more treatments are available now than before.