The rapper explained on IG Live Sunday she had her butt injections removed along with fibrosis removal in January after getting the all-clear from two blood tests -- but in March, when she discovered she was pregnant, her OBGYN gave her some shocking news.

Cardi said the doc dropped the bombshell ... revealing she was 17 to 18 weeks pregnant -- yep, that means she was expecting during her surgery.

She said the news hit her like a ton of bricks, 'cause she thought she was only 7 to 8 weeks along, thanks to a little Valentine’s Day action with Offset.

The hitmaker looked back and realized she hadn't had her period in February or March, but figured it was just the surgery's aftermath. However, once she started connecting the dots, she remembered the real baby-making night was New Year's Eve!

Cardi admitted she was sweating bullets her baby girl might have complications from the surgery, but thankfully, she gave birth earlier this month without a hitch. The baby's arrival came shortly after Cardi announced she and Offset were officially calling it quits.

Cardi’s divorce papers confirmed Offset is indeed the baby daddy of her newest little one, just like he is of their other two kiddos, Kulture and Wave.