I see the problem. You need to add some dolomite lime to the soil, or mix some powdered into the next watering and pour into the soil. You are growing in soil using organic nutrients, that means you need to maintain a pH of about 6.5-6.7 for healthy plants and soil. The dolomite lime (needs to be powdered not pellet) will buffer the soil to the proper levels and will provide some magnesium. You need to do this right away, about 2-3 TbSp in that pot and try to stir into the soil before watering (or mix with water then pour into soil)
You are seeing some lower "yellowing-off" which is not at all unusual when late in flowering, or when the leaves are low on the plant and not getting much light. But given the size and age of your plant, I would be more inclined to think the most of the problem is the low pH.