What Causes Rash On Inner Thigh And Top Of Back?

i have a rash on both of my inner thighs and the top of my back i only noticed the rash yeaterday and it is not sore or itching the rash just seems to be very small pimples and the skin just feels rougher round the area where the rash is but the rash seems to be speading further up my spine what could this be?