Check your bottling bucket spigot (if you're using a three piece). This got me and ruined a couple of batches in a row. **** gets stuck in between the two pieces and grows bacteria, infecting your beer. I know nothing is supposed to get in there, but it does. Run some hot water over it and pull it apart (believe me, it will come apart) and clean in between the pieces. I started using a two piece spigot that doesn't swivel for this exact reason.