Actually, I'll bet that the problem was that your plant was totally ready for a transplant and you are only just seeing the symptoms of a drop in pH in that old soil now. Since I'm guessing that plant did not double in size in 8 days, there's no way it WASN'T pot bound, and the slight downcurl of the leaf edges on that closeup agree with that.
Keep an eye on it to see if it gets worse in the next couple days, and make sure that you are watering with a fert solution that is in the correct pH range.
What brand of potting soil is that?
An easy rule to follow is, if you don't check your pH regularly, at least use a medium that is predictable, and pot up every 6 weeks. I am not a huge fan of peat-based (most are) potting soils, but one good thing is that they are always stabilized with lime at the factory, and you can expect them to behave the same way every time, as long as your water doesn't totally suck.