Leave it be :)

Who knows what caused that...

If you do not feed it fertilizer, you rely on the life in your potting mix to provide the nutrients. That life is going through its own cicles and stuff and even though it's all natural and organic and all, there might not be the right nutrients available at the right time, especially if the pot is small - just sayin ...

Organic processes in the soil might cause nutrient spikes or shortages that are totally out of your control.

Do you know about mykorrhiza?