Why is my gas meter going down so fast?

Ok so this started around a week ago I filled my 2003 Ford F-150 up all the way around 20 gallons it has 200,000 miles if that helps prob gets like 16 to the gallon I filled it up and was empty couple days later now I didn't drive any farther than I usually do and when I look at the meter and look at again later I can swear it has moved a lot. What is wrong? I don't think I have a leak cause I have checked, the other day it was leaking water I guess cause I smelt it and tasted it and I know it wasn't gas but I don't think I'm leaking gas, so is it just my meter or something or what it is there something wrong with my truck that it needs to burn more gas

Maybe someone syphoned your gas.