Dina Lohan: Hell Week in Hospital Bed With Mystery Illness

Lindsay Lohan will have to celebrate her 30th birthday without Dina Lohan ... who's holed up in a hospital and doctors have no idea what's wrong with her.

Sources close to Dina tell us she was taken to the hospital last Friday after complaining about severe stomach pains. We know what you're thinking -- it wasn't gas.

Doctors couldn't diagnose that initial pain, and we're told now Dina's aches have moved to her back and neck ... and it's getting worse. She's had several MRIs and CAT scans, but doctors are still baffled and Dina's spent the last 6 nights in the hospital.

The family sent us a pic, and while she seems to be in decent spirits ... we're told Dina's upset about missing LiLo's bday party in Greece.

Docs are managing her pain with meds.