positive chlamydia 6 weeks after a meet... how do i tell the couple

right so i'm the bad news today. went for my routine 6 week panel after a meet in glasgow and chlamydia came back positive. dh tested neg which makes sense, we've only been with this one other couple in that window and it was unprotected oral and protected pen. i'm mortified. i know it's not the end of the world, one week of doxy and it's done, but i have to tell them and i feel sick about it. we've met them maybe 4 times, i wouldn't call us mates but we're friendly. how do you word the message... i keep typing it out and deleting it. also is it possible i had it before the meet and just didn't know