Your plant is putting all of its nutrients into the flowering process, when you get this well into it, you can expect most leaves to begin to yellow. It just means your plant wants to make bigger buds! No worries!(This usually occurs starting from the bottom leaves to the top leaves)
Things that cause curling are: Over Fertilizing, High heat, To much light, over-watering, and under-watering.
Classic over fertilizing. Flush the soil with loads of fresh water then add a mild nutrient solution. About half recommended strength