What Causes Recurring Itchy Rashes On Pubic Area?

I have a flat rash that comes and goes in my pubic hair area. I used to shave and everyone seems to think it is razor burn. it comes everyday when I m wearing day pants and when I get home and take a shower and put on loose sweat pants the pain eases and by the next morning it is gone. Then it comes back the next day when I wear pants again. It is rarely itchy. It just mostly burns. The rash is not one big blob more like small separated spots. I no longer shave and it still comes everyday and leaves overnight. The skin is tender to touch even when the rash isn t present in the morning. This is the only place this occurs. Not in my lady parts but just the front pubic hair area. I also recently was diagnosed with low thyroid and have been taking medication but this rash has gone on like this for months. Could the rash be related to the thyroid problem? Or maybe contact dermatitis? The coming and going is confusing and it is not raised or blistery. It gets this way from the rubbing of the zipper part of my jeans.