What's this rash? Photo of side

[Watchwatchmymysteedsteedgogofarfar]

I've had this rash a few weeks. Seems to be very slowly spreading. Antihistamines make it itch less. I don't have allergies.

What is it? It's on both sides and in my armpits and down to the creases of my groin. Thanks in advance.

I have put sensitive image, but it's just my side boob.

Pityriasis rosea? My son had similar.
It got worse then gradually went after a few weeks.

My son also had this - started with a herald patch. I don't have that. I don't think it's this condition. Thank you tough, I had considered that.

Have you looked on DermNet? They have a diagnosis tool that you can use which might have suggestions.