A rep for Miller's doctor, Dr. Hooman Melamed, tells us ... her emergency back surgery Tuesday was to alleviate what he thought was an infection in her spine, but after doing some tests -- he now believes she is suffering from Burkitt Lymphoma ... a form of non-Hodgkins lymphoma.

We're told further testing is needed to confirm the cancer, but she should know for sure in the next day or 2. Regardless, Miller's doc says she'll need another spinal surgery, but is resting comfortably after her first operation.

When we last saw Abby ... she was leaving the halfway house where she is serving the rest of her sentence for fraud to get medical treatment. She was visibly upset and appeared to be in pain.

She's scheduled to be released from the halfway house May 25.