Is there something wrong with my discus?

I had discus, all of them are active and most of the time not hiding, but last week, I got 4 pots of cabomba and added them to my tank until it looks like green, thick, lush bushes. And then, about 3 days after I added the plants one of my discus is acting strange, it hid behind the plants for a great deal of time and will only come out when the room is empty for a while. When it's out, I sneaked quietly and managed to get close to check for some signs of illness, but there's nothing wrong, what wrong with my discus?