The reality star tells TMZ she had a routine checkup Monday, but relayed to her doctor she's been having severe headaches and dizziness as of late ... which was apparently enough to cause alarm, so her doc made the decision to admit her to the hospital.

Mama June tells us she's been in the hospital ever since her initial appointment, and has undergone a series of scans and tests, while everyone tries to figure out a diagnosis.

At this point, Mama June says she doesn't know what's wrong.

While she's struggled with addiction in the past, MJ tells us she has NOT relapsed ... and whatever is going on with her current medical situation is not substance-induced.