Doctor said my potassium level wasnt normal and eat more banannas. but what cause it to not be normal?

See below: The most common cause is the taking of diuretic medication.

Low potassium level: A common cause of low potassium (k) levels is magnesium (m) deficiency. Have your physician check your m to see if your level is low. Patients on diuretics loose not only k, but m as well which contributes to a low k level. Your k level will not increase until your m blood level is repleted.