EXCLUSIVE

Kelly Osbourne's seizure may have been much more serious than first suspected, because doctors say she could be suffering from epilepsy.

Kelly has been hospitalized since suffering a seizure Thursday on the set of "Fashion Police." Sources connected with Kelly tell TMZ ... the seizure was far more serious than first suspected. Doctors are running tests and we're told she'll be in the hospital for at least a few more days.

We're told doctors say there's a real possibility that Kelly is an epileptic, but it's possible it was an isolated incident. If it's the latter, docs need to figure out what happened and how they can prevent another episode.