I'm down 15 lbs since my surgery last Monday (Aug 27th). That seems like too much weight to lose in a week. Is this normal?

Normal to me, I lost about 10-15 in the first week. My guess is a lot of it is water, especially because they fill you up with IV fluids in the hospital.

Yes it's normal. Like CO said, It's mostly water that you're going to lose the first week or so. You're doing just fine don't worry!