Usually pityraisis is 5mm or so in size-
We don't know why it happens, but moissturizers make it better.
It does become more prominent in the sun because the rest of the skin around it darkens.
The patch does not darken. It starts red an sort of raised- then becomes whitish and raised, and then ends up flat and whitsh.
(SOUNDS EXACTLY WHAT YOU DESCRIBED!!)
If it is an eczematoid reaction to the mud, topical hydrocortisone cream 1% will help it resolve with moisture topically as well.
Eliminate the detergent if it is a possible offender.
The best moisturizer is called Lachydrin 12% which they sell at pharmacies without prescription.
1) 1% hydrocortisone twice a day
2) Lachydrin which you can apply very liberally 2 times a day after the hydrocortisone.
3) If the lesions do not improve, the possibility then is it is a tinea, or fungus.
4) At that point, Selenium sulfide 2.5% shampoo can be used on the body skin/scalp to
wash with, and thus help eliminate fungus...but you need a prescription for this.
But I am still going with the Pityriasis and use the Otc skin treatments (MUD).
I hope this was helpful=
Feel free to ask anythng else you need.
Good luck, and be well.
My very best--