Clearly if you are sick, not only is it “ok” to cancel, it’s the moral thing to do to prevent infecting the provider with your flu bug.
i get the anxiety of being labeled a “flake” for a cancellation. If you don’t already have a relationship with the provider, I’d suggest sending him a cancellation fee/deposit. 50-100 bucks as down payment on your rescheduled future appointment to assure him your cancellation was in good faith and to assuage the inconvenience of the cancellation.