Ive been getting alot of buttne out of nowhere! Help!

So i have some suspicions, i got it right before my period started which might be the issue but i already have like 10 red tiny pimple like things on my butt, it could also be from a new underwear o bought. It has a thin lace at the back where my butt sits. Anyway does anybody know how fast it goes away and how to treat it without actually spending money?? Im really broke so i cant buy a ton of products.