Don't know what's causing

OK my friend is that coco or soil as if soil defo not calmag trouble and if only on bottom leaves look at your new growth as leaves are a time line meaning if your new growth is lush and green your problem is a thing that was wrong and now your fine. If your plant is bushy uptop then look at how much light is getting at your problem down low if is not getting light then undergrowth does yellow and fall away. I would say if is recieving light and new growth is nice then at the point it yellowed your nutrients where to low and as you said its only down low you have now dialed in your nuts correctly and got no drama. So don't worry my friend.add me to your network and I be happy to help anytime. What we do is an art lets help eacother creat masterpieces.