How do I call in from work

I recently got diagnosed with Lupus and it has been very active. Right now I think I have the flu. My throat is dry, i keep dry coughing, and vomitting plus im light headed but thats probably because I haven't eaten much since it started last night. I plan to see my doctor at 4. I am scheduled to work at 5 30. I am unsure of how to tell my manager. She knows I have Lupus and i recently put in my two weeks notice since my parents want me to focus on my health. I once had to call in because my knees were very stiff and it was painful to move. I feel like im constantly being unreliable but the disease is really kicking my butt. But thats not their fault.