POOL PARTY NEED HELP

ok so I'm going to a pool party tomorrow and all my boyfriends friends who I have not exactly met will be there
And I have this rash on my ass
And idk what to do
And I know that I will be made fun of if they see this rash on my ass
I get it if I sweat a lot and it's these little spots that like cluster up in a few spots on my thutt
It's noticeable.
My dermatologist gave me this ointment but like I haven't been using it much lately cause I didn't know about this party till last minute
SOS how do I cover it up!?