Are you on a 'proper' fast - ie water only, or on an extreme calorie restricted regimen, ie fruit juices, veggie smoothies, etc?
FYI, I weighed myself this morning in my kitchen. Two hrs later I weighed myself in my bathroom, and registered a 1.50lb 'loss'. For all practical purposes, that is virtually impossible.
Scales (even digital electronic) give only an approximation of mass. Weight is a function of the earth's gravitational 'pull' on your body. Depending on your precise location, time of day, etc., it can vary within certain parameters - possibly by as much as 1k. General trends are more significant than a single reading - or two.