You bring up a couple of good points.  First, increased pigment can occur lots of other places besides the face and second, it can be just as annoying and cause self consciousness!   Sometimes increased pigment in those areas IS just the result of rubbing but sometimes it's related to other more serious problems.  Please see your doctor and be evaluated, at the very minimum, for early diabetes, thyroid problems, and other hormonal changes that can cause increased pigment in these areas. 

If it turns out it's just due to rubbing, your doctor may prescribe something for you but also really try to protect the area - sometimes vaseline or a body butter applied first helps and make sure your clothes fit well (spandex is much better than shorts, for example).    Hope this helps.  Dr. I