Same thing happened to one of mine... not trying to scare you but one of my males had it and it got worse and worse to the point where he could barely open his eyes.

He started looking skinnier and skinner and one day he just got so weak he could barely move. He's the first rat that I've ever had die on me... (except when snakes ate ofcourse)

And it just happened not too long ago, like 3-4 days ago.... I have no idea what may have caused it... But he is house with another male but I've never seen them fight. All of my other rats are fine.. I feed them good quality mazuri rodent diet and they always have water....