Dear Johnny,
Thanks a lot for scheduling my interview for next Monday; I am very excited.
Unfortunately I got sick today, and I have to see the doctor next Monday at the same time as the interview.
The interview is very important for me, and I wish circumstances were different.
I am writting this email to ask you to postpone my interview to a later time next week. I apologize for the inconvenience.
Please let me know the new schedule after you decide.
Thanks for understanding.