You say that your husband had a tumor removed from his lip, and you've experienced some discomfort since then after oral sex. The only possibility I can imagine that's specific to your situation is if he's using topical chemotherapy, a scant amount of which could be transferred to you.
It's more likely that the saliva is an irritant. As our vulvar skin becomes more thin and fragile, it can be irritated by enzymes in the saliva. You can use warm water--you don't need soap--to rinse after oral-genital contact. And you may find more comfort by using a lubricant like LiquidSilk or Yes (which is also a moisturizer) at the opening of the vagina and surrounding area.