Mares & heat cycle question

I've been meaning to ask the vet about this (and still will), but thought since I'm thinking of it now, that I'd post here and see if any of you wonderful folks out there have seen this behavior.

One of the mares (6 year old maiden) has been periodically rubbing her hind end raw. She has no discharge to indicate a yeast or other vaginal infection, and there's no smell. She's wormed regularly, and she's the only one of the eight rubbing. She started this last spring, and my initial thought was worms.

During the winter the horses are kept mainly in the upper lot and small front pasture. They scavenge after dropped grain under their feed buckets and on the ground/mud around the corn bin, so picking up worms wouldn't be unlikely. And it certainly seemed like worming her took care of the problem - but only briefly.

I finally put two and two together, and timing-wise, I'm pretty sure she's rubbing only when she's cycling.

She's simply determined to rub for about a 1 week period (long enough to scrape most of the hair off her butt on either side of her tail and make a nice raw marks), and then she stops.

Washing her hiney off with iodine soap or betadine solution, and then applying salve - Corona or Nitrofurazone - seems to help somewhat. (At least it minimizes the amount of chafing & hair loss.) She doesn't get cranky, has a good appetite, and in a little less than a week she's obviously not uncomfortable or itchy anymore.

I've seen mares colic (and I thought I have it bad when I get cramps!) when they start to come in season, and I've seen all sorts of degrees of "mare-ish" - but this is a new one on me. Anybody have any thoughts??