Did you happen to notice any signs of stress before you transplanted? Was the plant quite rootbound? I wonder about the possibility that it is now showing signs of previous suffering.
The spots look vaguely fungal to me and a treatment with Serenade (cheap at walmart or home dePOT) is not a bad idea.
If it's just a lower leaf or two and does not spread in the next day or so, it's probably from before. The color of the spots is too chocolatey to be potassium deficiency. I'm sorry that one has me stumped at the moment.