I’m male. Another male I don’t know got into spanking my butt cheeks with his hand repeatedly very hard. My cheeks got very red but I saw zero evidence of bleeding afterward. 

Welcome back to the forum. I’ll be glad to comment. It sounds like the encounter you described was not pleasant, and your partner may have gotten carried away. Despite that, there is no risk for any STI from the spanking/trauma you experienced, even if he had genital secretions on his hands. There is no scientific reason for concern or testing. EWH.---

Thank you!  I even have some bruising so I was worried about micro cuts or something. 

Even if there were bruising, scrapes or small cuts this would still be a no risk encounter.  EWH---

Thank you so much for putting my mind at ease. 

That you were willing is fine.  Don’t beat yourself up for your actions.  I encourage you to continue to practice safe sex as you have.---