Help .. can my period just be irregular and the pains is because of hard penatration ? Or std?

Chaiy

..I’m 17 and today made 5 days of a missed period …. I don’t think I’m pregnant because there was not sperm that I remember on the 12th.. or the 8th .. I had a new sex partner today and I burned after I peed like normal but now I’m having abdominal pain n lower back pain … what does this mean honest answers only .. idk if you can see std/sti signs a few hours later