it would bethat it should be an STD
or something else. Causes of painless eruptions around the buttocks, like non itching are also found in diseases like syphillis and oether simple viral, bacterial and allergic reactions. particular non cotton pants could cause your system to react to them and produce such symptoms. If the eruptions produce pus when burst open then a bacterial origin would be likely. Would be interesting to know if you have any difficulties during urination or not or any recent skin eruptions around your kidney
Visiting your primary care
physician for a consultation and management is worth doing. I believe its really too early to think its an STD now.
Thanks as I wish you the best and stay well,