Prolonged sweating, infections, drinking too much water, diabetes insipidus
, adrenal gland dysfunction, kidney
and liver problems
, prolonged diarrhea,or vomiting, certain medications etc can all cause low sodium in the blood. This is a serious issue and needs to be investigated. You need to consult a doctor and run tests to find the cause.
Treatment is to increase fluid—not water—intake. So drink clear soups, lemonade, juices, oral rehydration solutions etc.