Thanks for writing to us.Clotrimazole
worked on your back as you had an active fungal infection
Now over your thighs what you have is post infalmmatory hyperpigmentation
. This area of body is prone to friction and sweat and therefore chances of darkening of skin are more. Here clotrimazole an antifungal cream would not help.
You may require application of pigmetation reducing creams like kojic acid glycolic acid
or hydroquinone to reduce the pigmentation at the mentioned site.
Hope that helped.