In case you missed it ... Kelly appeared on the 'Off the Vine' podcast earlier this month, where she recalled how she landed her now-husband when she was a cheerleader in Athens. During the show, she admitted to going after his backup first to get his attention ... and, of course, the plan ultimately worked.

However, the social media commentary afterward was not about relating to Kelly and Matthew's slow start ... it was more on ID'ing the unnamed backup -- and many assumed it was Joe Cox.

With the story spreading like wildfire, Kelly finally broke her silence on Tuesday. Over on Instagram, Stafford clapped back at the haters who claimed she was "embarrassing" the Rams superstar with the story ... 'cause she insists Matthew doesn't care one bit.

She also sent a heartfelt message to Cox and his family ... sharing her regret for pulling them into her mess, especially considering Joe was NOT the backup QB she was talking about.

As it turns out, Joe was already dating his now-wife when her whole story took place ... and Kelly says his relationship served as inspiration for what she wanted with Matthew.

Neither Joe nor his wife have publicly commented on the matter, but odds are we may never hear from them ... as they've seemingly remained pretty quiet over the past few weeks.