I agree with BirdDancer -- you should talk a little bit about your diagnosis if he's already noticing things, especially with your medication, unless you want to keep things casual. If he's noticing things, then he may suspect you're hiding things from him, which is technically true, and that can show that you don't "trust" him. (I'm not saying you don't trust him, but from his POV, he may go as far as thinking you don't trust him with anything.)

Also, BirdDancer is right that you need to speak out about having a bipolar Dx if you are married or living together. He at least needs to know at that point that you have bipolar. But the longer you hide things from him, the more suspicious he may get, and the more he may lose your trust. You know, "Why didn't you tell me earlier???"