Yellowing of older or "bottom" leaves may simply be a sign of age and the natural growth during the plant life cycle.
Check the new growth or "upper" leaves for signs of yellowing along the veins. If there is no yellowing and the new growth appears healthy, it is very likely you have natural dying of older growth leaves and have nothing to fear. This process is essential for the plant to maintain it's strength as it shifts nutrient uptake to the new growth.
Relax! But keep an eye on new growth leaves for any signs of discoloration. Let those bottom leaves drop off and pop them into the compost pile!
More to come.