Teddi Mellencamp Reveals Doctors Found Multiple Tumors on Brain

Teddi Mellencamp's brain is covered in tumors ... and, doctors are working quickly to remove the biggest ones, she revealed Wednesday.

The reality television star shared the news in a simple black-and-white text post ... telling her millions of followers she's been out of commission the last few weeks due to a series of debilitating headaches.

Mellencamp was fighting through them until Tuesday, she says ... when the pain forced her to go to the hospital. Doctors then administered a CT scan and an MRI -- and, Teddi says they found multiple tumors on her brain, which her doctors think have been growing for months.

The former 'Real Housewives of Beverly Hills' mainstay, and daughter of rocker John Mellencamp, says she's having surgery today to remove the two biggest -- but, the smaller ones will be dealt with via radiation further down the line.

Mellencamp ends her message by saying she's grateful for all the healthcare professionals working to help her get better ... and, to all her fans who are offering support.

BTW ... Mellencamp also shared a clip where she's being prepped for surgery -- shaving off her hair so doctors can cut into her skull.

Worth noting, Mellencamp's dealt with cancer before. Just last week, Teddi shared a photo of a huge cut on her back -- explaining she started to develop little sunspots years ago, and she ultimately needed to have numerous surgeries to attack the melanoma.