It's entirely possible it was a "fart egg" (or "wind egg"), which is when it is basically a blooper egg. It is often small and without a shell, and our hens seem to not notice laying them and we often find them on the roost. Perhaps it doesn't feel like an egg to them.
It is possible, of course, it is nothing to be worried about and as spook said something harmless like a slug. But be careful and keep watching the poops. If there is one particular bird with the wormy poops, quarantine him or her immediately. And (pardon me if this is incorrect, as I said this...