Even without infection, diacetyl can occur after packaging. Diacetyl forms in fermented beer once the beer begins to oxidize. If you are getting oxygen into your system on the hot-side or post boil, diacetyl can appear in packaged beer even if you don't detect it when packaging.
Having said this, it sounds like you have a nasty case of it, and I agree with Mike that infection is a likely cause. Pediococcus bacteria is a common culprit.