Worry not my brother, you aren't losing muscle mass as long as you are getting your bodyweight in protein and doing the proper re-feeds.
Your weight lost is not all fat. It is water and glycogen. Glycogen is actually pretty weight heavy. You've probably lost 1-2lbs of fat, 7-8lbs water, 3-4lbs from glycogen depletion.
When you are depleted like you are, you are going to look flat and bare. You will feel like you are burning muscle but you aren't.
If you are cutting your cals that severly, you need to make sure you only do it for like 4 days max.
What are you doing for your re-feeds? and how often?