Tell us exactly what you did yesterday and how you did it please.
When a fish changes as quickly as that, from being normal to "definately something wrong" AND you have very recently done something to the tank, you dont usually go far wrong assuming it was something you did while you were fiddling with the tank.
Immediately, I would say there's something wrong with the water, but it depends on the plec, how old he is, what type he is etc. For example, if this is a brand new plec and this was his first water change at your house, then he may be nothing more then a bit freaked out. However, if it's an 8 year old that you've had for a few years, then something in what you did has probably made the difference.
Tell us about the plec, the tank and how long you've had him etc please, also how he's behaving now, any change?