Was it a large container? You said you served some out of it.

I think that once you've opened something, it's got a limited shelf life, since the seal of freshness has been broken and allowed air into the container.

I've bought the little one serving containers of Yoplait, and without even opening the container, opened it up a day or two later to find that it was somehow compromised and no longer edible, even if I didnt open it up till I was ready to eat it.