Burm refusing to eat..

[Squirtle]

This is the first time EVER my burm has refused to eat.. Nothing in husbandry has changed (same size enclosure, water changes every two days, temperature are at mid 80's since it's winter). His last meal was about three weeks ago, and I've fed him every two weeks up until now. Anyone know what's wrong? He just turned two years old December 20th.. maybe he wants to breed? Should I be worried? He's almost 9 feet long and 40 pounds.