I would weigh again tomorrow and see if it still shows a 5 lb loss. There could be many answers to your question. If it is true body fat loss and not water, you will show a consistent 5 lb loss for the next week everyday varying by about 1-2 pounds . Maybe it was just that particular outfit that felt tight. If your clothes are only tight in the belly area, maybe that "time of the month" is coming and your truly lost 5 lbs of fat. You never know.... The scale can be very misleading at times.