There was quite the awkward showdown on last night's "Teen Mom 2" reunion -- where Kailynn Lowry accused ex-boyfriend Jo Rivera of giving her an STD.
During Tuesday night's special, Kailyn revealed she got tested after cheating on her boyfriend Jordan with her ex.
Lowry tested positive for Chlamydia, but Jordan was negative ... prompting Kailynn to say "that leaves one person to have left an STD to spread."
Jo took a minute to catch up ... eventually asking her "Wait, did you just blame me for giving you STDs?"
That led to an tense conversation between Kailynn and her two men, which Dr. Drew Pinsky ended by saying "Chlamydia can be very confusing."
Check out the showdown above.