That's Brittany Matthews -- Patrick Mahomes' fiancee -- getting pretty candid about her pregnancy ... revealing the two weren't planning on having kids before their wedding, but they consider it “the biggest blessing [they’ll] ever have!”

Matthews -- a fitness pro who's been dating the KC Chiefs QB since high school -- says she was on vacation when she found out she was preggers ... and almost didn't realize it at first!

"It's funny because I read the pregnancy test wrong at the beginning and I thought I was all good to go -- ready to party, vacation ya know let's do this! Then, I had a weird feeling and I went back and checked just to make sure and it said positive."

Brit admits she "freaked out" -- but notes Patrick was "just so sweet and supportive" and "things have just been great ever since."

No, they weren't "trying" to get pregnant -- but Brit says they weren't exactly "being careful either."

Britney and Patrick got engaged earlier this year -- and she was asked on IG Live if they'll try to speed up the wedding to tie then knot before the baby is born.

"Actually, had a meeting about this yesterday with our wedding planner," Brittney said.

"We have a few dates nailed down but we'll be having baby in 2021 and we plan to do a wedding and all wedding things in 2022."

In other words, SHE'S FREE TO BOOZE AT THE WEDDING Y'ALL!!