How should I phrase it correctly?

Here's what happened: I was admitted to the hospital last week (due to an accident at work) due to solvent poisoning. I was then discharged because my symptoms had subsided (I was on my last night shift and was on the day shift at the hospital). After that, I felt better again. I told my boss I didn't need any sick leave, but now the symptoms are back, and I'm worried he'll misunderstand me, especially since I'm still on probation. Can anyone tell me how to phrase this correctly?