These sound to me like they are potentially stretch marks, which are very common on the back of the thighs. Stretch marks occur when the the skin becomes stretched, often under the influence of a change in hormone levels (this is why, for example they are more common during and after pregnancy). Initially they are paler than the surrounding skin and may be reddish in color but, as they heal up, they darken and may also thicken, giving a ridged texture.
Stretch marks do not usually require medical attention, as they tend to fade over time and not be a major cosmetic concern if they are on the back of the thighs. Creams and lotions marketed to help with stretch marks do not actually help very much.
Occasionally, stretch marks can be a sign of an imbalance in steroid hormone levels in the body, called Cushing's syndrome. I advise you to discuss this with your dermatologist
. He or she can do a full evaluation, and look for other symptoms, such as stretch marks in other parts of your body, fluid retention, new hair growth, etc. Good luck.