Monday night was a real pain in the boob for Nikki Bella ... 'cause she says one of her breast implants got stuck in her ribs after a recent in-ring showdown!!
Nikki opened up on the boob mishap in a convo with her sister on their "The Nikki & Brie Show" ... saying it happened on "Monday Night RAW" this past week during her 8-woman tag team match.
"Piper's elbow drop? Yeah, put the implants right into the ribs," she said. "It's been hard to laugh and cough, but it's okay, we're gonna get it fixed one day.