I Would Get The Cooling System Pressure Teated To Determine Where The Coolant Is Going. You Could Have An External Or Internal Leak.
With an internal leak, coolant can be consumed by the engine and will not be noticed until the coolant level drops.
I repectfully disagree with advice given pertaining to heater output. Low coolant is one of the first things to check when a heater starts blowing cold. You have experienced this situation yourself, twice.
CSA