The same thing happened to us...
The Windsor Humane Society put down our cat less than 48 hours after they took her in, and she was nursing kittens. We went down after the first day she was missing and they said they had never seen her. The only reason we found out is because one of their volunteers saw our missing/lost ad in the paper and called us to tell us she was put down. I was a total mess. I'm truly sorry about your loss.