Please send photos for accurate diagnosis. Gauging from your limited words, you have probably developed acanthosis nigricans. In this condition all the fault of the body like back of neck armpits run in respect of thighs become dark. Please get your
sugar and cholesterol levels tested kindly start exercising as this is a direct result of being overweight or because of sedentary lifestyle. This can be treated through creams and chemical peels. Please contact me with photos in private chat for prescription.