First time Oral Sex

How do you tell a guy you don't want to get oral from him for the first time because you think you might have a yeast infection?

Update: I did end up telling him or more like jokingly made him guess it. He took it well, wasn't at all disgusted and knew it happens to girls once in a while. But why is it that men are always so quick to ask whether you wash yourself correctly? There are many ways to easily get a yeast infection, lack of hygiene isn't always the case. I just have a sensitive vagina sadly.🤦‍♀️