Chronic Heat Rash or Dermatitis Herpetiformis

Can you please share some of the distinguishing factors between a chronic heat rash and dermatitis herpetiformis?

I have been struggling with an extremely itchy, chronic rash that comes for months at a time, then goes, and returns again after a period of clearing.  It leaves some darker pigmentation after it clears up.  The rash usually starts on my chest and also appears on the backs of my elbows/upper arms, lower back/buttocks, outer thighs, and hairline (all within the same breakout).  I don't get it on my knees, hands, feet, or face.  The rash is a pinkish warm area at first, and then becomes more organized and bumpy (small bumps, not large ones).

Heat, friction, and dry skin all seem to aggravate it, especially heat and moisture.

I've had the rash at all different times of the year.  Nothing seems to help significantly, i.e. Cortisone cream, Benadryl gel, lotions, etc.  It just has to run its course and finally go away - months later.

As I mentioned, heat really aggravates it, but can a heat rash really last for months at a time?

My dermatologist mentioned dermatits herpetiformis, however, he is not sure.  We are keeping an eye on it.

I am in my 30's, and this rash started about five years ago, and it's been on and off ever since.  I'm of scandanavian ancestry, and I have been taking a progesterone-only oral contraceptive for the past five years.

Any thoughts would be greatly appreciated.