Shannen Doherty -- I'M NOT PREGNANT ... I Just Like Burgers

"90210" star Shannen Doherty HAS A BUN IN THE OVEN -- but it's not the baby kind ... it's the burger kind.

Rumors have been swirling today after a photo of Shannen emerged ... in which her stomach was a little more accentuated than usual.

But the actress says there's DEFINITELY NOT a baby in her belly ... just lunch.

She wrote, "I'm a size 2, not sporting a baby bump. That's simply the hamburger I wolfed down 2 minutes prior."

Shannen's little bundle of joy is expected to be delivered in the next couple of hours.