Are you on a liquid diet? Did your doctor give you a pre surgery goal weight? My surgery wasn't approved unless I met the goal weight. My period started about two days before my surgery and I gained a little, (They still did a pregnancy test the morning of...) but I was still under my goal weight. They didn't weigh me the morning before the surgery. I had a pre surgical visit the week before where I was weighed and I believe I had gained 3 lbs. There is generally always a difference between the scales and time of day.
It's a odd/stressful time before the surgery. There were so many times where I would imagine a reason why they might cancel the surgery, and it was all in my head. Try to relax-- breathing exercises, find positive mantras, remind yourself why you're doing this... :-)