That's really weird, the same thing happened to me. I transplanted like 4 days ago, and the older leaves are turning yellow, and there's some gray/brown showing up on the leaves. I decided to use fragrance/dye free dryer sheets to get rid of the fungus gnats, because I thought that might be what caused it. Then after looking online, I read that it could be a Potassium problem...and I watered with some ferts. Hopefully this will help my plant...if it does I'll let you know and see if the same could be done with your plant..but if things are looking up I probably wouldn't worry too much about it. Maybe it's just transplant shock. Ah well, good luck!