teeny tiny soft duck egg

Okay, this is kinda weird but here's what happened.
Just now, as I went to the coop to call in our birds, I saw our cayuga drake mount our cayuga duck out in the yard. This is quite unusual for them to mate out of water. When they were through I noticed on the ground on a wet spot a teeny tiny egg. i do have a picture, but not sure how to load it. It's about the size of a peanut butter M&M. it's opalescent and a bit soft-shelled on one side. It has what appears to be a small spot of yellow yolk inside too. What the heck happened?! She appears to be fine, though a bit dazed. She let me approach her and look her over, which is unusual too as she normally skirts away.
Anyone ever known this to happen? It popped right out just as he withdrew.
Any insight would be appreciated.
Thanks!

 

So the image of the teeny tiny egg uploaded successfully, but do I need to link it to my post now, how would I do that?
Sorry I sound so unfamiliar with these things, i haven't posted much
thanks