I made an all grain amber 2 weeks ago. Fermented in the primary for 1 week, and the beer tasted great. Transferred it to the secondary for 1 more week and last night I went to keg it and it had a sour taste. I am always very careful of keeping everything very clean and sanitized. I ferment in 6.5 gallon plastic buckets. Could this be the cause of the problem? Thanks for the help.