I can't comment on whether or not food or exercise affects the reading, but I can say I have never been advised to avoid either, in advance of my monthly tests. Don't forget that the e, in eGFR, stands for estimated, and so it can vary for numerous reasons. The calculation used can vary across labs and is influenced by numerous factors including age, so as that has increased since your last test, could in fact be one simple reason for a different reading.