["I left work early today and feel a little bad about it. I was really sick and now I know I'll have to play catch up later today when I go back."]
Oh, you shouldn't feel bad! Everyone gets sick every once in a while and I would hope that your boss and coworkers were very understanding of your needs. It is important to take care of yourself.
I shouldn't have left work early today but I was feeling super ill. I know tonight I'll have to play catch up with my work.