The former "90210" star revealed the news Tuesday night saying she underwent a battery of tests ... revealing one of the tests yielded good results while the other came back "elevated."

Shannen -- who almost a year to the day went into remission after battling breast cancer -- said the results mean she'll get monitored even closer. And to prove she's staying positive ... she said, "the elevation can be from all sorts of things. Oh and vitamin D...need more of that too."