I'm assuming your T can't change your appointment day? I very rarely cancel which is a miracle considering my health issues (although I have forced myself there on many occasions I admit) but when I have done she's able to change the day/time just for that week. If I was in your situation I would ring her and say I don't think I'll be well enough to make Monday, is it possible to change it to Tuesday (or whenever).

I don't know if your T can/will do this though. My former T wouldn't have been able to.