yellowing overnight is usually an indication of stem rot (aka crown rot). yank it out of the pot NOW and clean off all the media--if you have any black sticky gunk down there (or if the rootball simply falls off), you have two options: toss the plant, pot and all, or try and salvage it. buy a few bottles of H2O2 and prepare to dunk the plant in it several times over the next few weeks. gimme a sec...
http://www.firstrays.com/remedies.htm
see treatment for crown rot; works elsewhere on the plant too.
whatever you do, DON'T handle any other plants until you've thoroughly washed your hands--rot spreads easily.