Dear xxx,
I don't think I can make tomorrow's appointment as I am having a really bad cold at the moment.
I am so sorry about that.I went to see the doctor yesterday and got some medicine.I thought I might get better today,but I still cannot get out of the bed now.I am having a terrible headache,and coughing badly.
Can we rearrange the appointment?I will be free from next Tuesday to Friday,and I think I should be completely recovered by that time.I promise you I will see you next time.
Best wishes,
xxx