STI confusion (read description)

I was with my last partner for 6 months and half way through our relationship I had an STI check that came back all clear.

They did not have an STI check - only I did (I know this might have been wreck less so no negative comments please - just curious for answers).

I know they did not cheat so there is no concerns there.

I am wondering if you guys think that when I got the STI check whether that means we were both clean or (if they had an STI) I just had not caught it at that point?

Feeling confused! Help!