Leaves turning yellow can mean a lot of things, starting from natural process of senescence (old leaves). Flower drop combined with leaf yellowing may indicate:

• Plant has outgrown the pot. Needs more media in a bigger pot
• Check drainage holes (usually covered with filters, like brick pieces / broken tiles etc) as water logging because of poor drainage and lack of aeration to roots can result in this combo.
• Every symptom need not be associated with pest / disease, as simple signs from plant can tell you it is not happy about something.
• Look for that indicator and set that right. Good Luck.
• Valliappan from YouTube Channel: Flower Valley Dr K Valliappan