Check to see if you have ice inside your freezer section along the bottom in the back. Most likely your drain located behind the rear panel is plugged and the water from the defrost cycles is backing up and freezing. Once the ice gets big enough, the only place for water to go in through the air returns down to the fresh food section. If you find ice, remove as much as you can so you can remove the rear panel. Continue to remove the ice using hot water and/or hair dryer until you can get to the drain. Poor hot water down the drain and use something to fish down the drain into the condensate pan under the unit until water flows freely. Then put it back together and dry everything with a towel.