Is there any discharge from the eye? Is Jack eating well? Has he had any injury to the eye? Third eyelids that come across may mean there is either injury, or can be early sign of them running a temperature. You could keep a watch on him for a day or so to see if it goes back into place, or get the vet to check him over.

As a side note, if the Fucithalmic had already been open before using it again this time it should really be thrown away if open for more than 28 days. With eye medications in particular its best not to use them longer than needed.