Kelly detailed her journey on her "The Morning After" podcast this week ... saying she always wanted to have bigger knockers, so she saved up for years to make it happen.

"I just turned 19," Stafford said. "So I went and got a boob job."

Kelly -- who started dating the L.A. Rams QB at the Univ. of Georgia -- said her man was opposed to the move at the time, but she went ahead and did it anyway.

"He was like, 'Do not do it. Please don't do it,' and I was like, 'Listen, love you, but I've wanted this for a long time and this is about me, not about you.'"

Maybe Kelly should have taken his advice ... 'cause she said it ended up being the worst boob job ever.

"It was two balloons on my chest," Kelly said. "It was so, so bad."

Kelly said the botched job was so obvious, Detroit reporters caught on and believed that her then-Lions QB boyfriend must have paid for her implants -- which pissed her off.

"I am independent. I don't need him to buy my boobs. I bought these boobs."

Kelly added, "If he would've bought 'em, they would've been far better!"

Thankfully, the 32-year-old mother of 4 was able to get them "deflated" a year later, and then eventually redone altogether just a couple years ago.

"They were so bad that it was very evident ... like that looks real bad."

Kelly -- who married Matthew in 2015 -- says even though he was against it, he was very supportive and "sweet" throughout her struggles.