why and I'm so uncofortable and my back hurts badly since I've quit taken percocet 20mg 4x daily to only percocet 5mg 4 x daily. Irritable,anxious,panick attacks,back pain.

Obviously because the drug made you feel better, but your doctor now feels that it is time to taper the Percocet down. It's easy to get addicted to all the prescription painkillers. If the pain is too much, you need to discuss this with the doctor. We have no way of knowing what the situation is.
This is a common and difficult subject. Some people just want more and more painkillers or want them forever, while others are in genuine pain. If the pain can't be 'seen' in test results, doctors are reluctant to over prescribe. They can lose their license!