What Causes Darker Skin Tone After Using Niltan And Suncros?

I am 29 years old girl having smooth, silky normal to dry skin with a wheatish to dark complexion. I regularly use Suncros aquagel spf26 from ranbaxy. My dr suggested me to use Dr Reddy's Niltan cream(in night only) for improving my color. But after using Niltan at night & Suncros spf in day , I became even more darker in just a week. Why did it so? Plse tell me the correct way to use Niltan OR plse suggest me an effective natural skin lightening cream which I can always use. Also I turn very dark as soon as I step out even on using Suncros. Plse suggest me an effective daily skin care routine with the creams' name.