when to tell employer

I'm 6 weeks tomorrow and suffering terribly with nausea, if I eat little and often it's fine but it's going to affect work, I worked 3 hours today and halfway I felt so sick....I have a 6 hour shift tomorrow and normally I don't eat and can go a whole shift without eating a thing, feeling like this I'm gonna have to eat something, just don't know how to explain to my manager as he knows I normally don't eat during work...