What does it mean if you feel fine one day and sick the next?

Alternating "fine" then "sick" may point to Allergies! But if so, you need to do some sleuthing to find out what you might be allergic to in food or the environment. Keep a diary of how you feel and where you went.

For example, I would get "sick"-- feel lousy, feel stuffy, my voice would deepen, and I'd feel like I had the flu. I noticed it happened if I ran the vacuum cleaner! 15-20 minutes later, I was "sick". When I went out on a breezy day, I got "sick" by the next day. If I touched a dusty object, I got "sick" immediately. I am allergic to--- dust and mold.