Officially normal sodium
(Na) is 135-145 mEq/L.
"Low 130s" implies your might be something like 131-134, right? Exact Na level varies pretty widely over the course of the day, and this amount of variation probably is entirely normal. If you have no other evidence of heart, kidney, or liver disease, this probably is nothing to worry about. If you do have such problems, discuss with your doctor.