Small pimple like rash?

So i have been at a ranch for the past couple of days doing long horse-rides for 2 to 3 hours at a time. Today, ive found small pimple like bumps of my butt ( i know it sounds like ive been "unclean") but ive had a shower every night and im sure to be clean. Anyways, it looks more like a bunch of little acne on my butt, which i have never gotten before and its in a way that it looks like a rash almost. Now i do sit of a sheepskin cushion for my saddle and i was wearing cheeky underwear with lace, actually most of it was almost in line with where the lace would be on cheeky underwear, and then some more towards the back of my thigh. But im thinking it might have something to do with that, but im not sure and im a little worried. Does anyone think they know what it could be? And why it popped up randomly? it wasn't there this morning..