"OOOooo that smell! Can't you smell that smell???"
My thought was like joellelee's, check the drip pan under the fridge.
If your fridge is self defrosting, it drains down into that pan. If it doesn't evaporate correctly ( something blocking the fan usually) it will stink!
I haven't yet figured out how to get the pan out, not even sure if it does come out, so I move the fridge and vacuum all around it and underneath it and inside the coils and use an extender to get all the way inside the guts.
Last time this happened to me, I moved the fridge and found a moldy puddle of something unidentifiable under it!!!
Good Luck!!