When the sub is sick

Last Thursday I was sicker than I can remember being for a long time. It was compounded by the fact that I had to do some legal work, go to class, and then go to a concert (the Decemberists). Also, I knew I had to sub the next day. The pressure of trying to do everything while I felt like crap really got to me. I was able to finish the legal work. I got to class a bit late, and was planning on going to the concert. I even told my professor that I would I have to leave early to go to the concert. About an hour into class I felt so sick that I almost couldn't bear it. The thought of going to the concert was too much, especially because I didn't feel like I could cancel subbing the next day and I knew I would feel worse if I didn't get enough sleep. Finally, I had to call my husband and cancel going to the concert, and then left class early. I am one of those people who cry when I feel really awful, so I almost started crying in front of my professor as I told her I had to go. That would have been embarrassing!

My big question - if you agree to sub in advance, can you call and cancel? Can a sub call in sick? I was so afraid if I did, that I would never get called again, so I went. Things ended up going well, but what if I felt as bad Friday as I did on Thursday? There would have been no way for me to have survived.