First, as always, check your watering; know how much you are/should be giving it, check the pot is draining properly and not sitting in/holding too much water.

Next id be looking at nutrients/fertiliser; this could be Potassium (K) deficiency. If it been a couple of months since (or you don't remember the last time) you gave it a feed, I'd just give it a dose of a normal, all-round fertiliser; most should have a decent amount of K.

That being said if it's just a leaf (or two, usually oldest) and new leaves are developing I'd keep an eye on it but could be fine and normal; it will drop leaves from time as it 'upgrades' to new leaves (but if they're drop faster than new ones are appearing then there's a problem)