this seems to be becoming a greater problem as of late. i think it may have to do with a bacterial infection. when you take it to the vet, see if he/she will lance it for you if they cant provide any answers. i had a couple of phantasticus come down with this condition. after the first one died, i carefully lanced the eye on the second, and it returned to normal. this could be tricky, though.
care to try to get some shots of it?