Methinks nitrogen deficiency. You could probably go ahead and start feeding with a low strength fertilizer now, and gradually work up to near full-strength, or until your problem is solved. Keep an eye on it, as long as the yellowing of leaves doesn't progress up the plant, just be sure not to overfertilize. Once you notice that the yellowing has subsided and new foliage is vibrant and green, cut back on the ferts until you need them again.

Then again, I'm partial to occasionally adding nitrogen-rich organic ferts like pulverized blood meal & fish emulsion while vegetating, and rarely experience signs of toxification. I also flush the soil at least once per month, to wash away excess salt buildup and to test my pH.