It doesn't sound like any sort of STD. If you've researched diseases like herpes, warts, etc, you probably already know that the presenting symptoms don't look or act like a regular pimple that gets a head and then breaks. I think you have... pimples

Your friend is probably on the right track in regards to friction and maybe sweat and oils causing them. It is also true that skin lesions would not typically appear the next day for most STDs.
No one here can diagnose you, of course, but I think you can relax