So recently I have been having some overheating problems, so I serviced it for a new thermostat, radtiator cap, and replaced a worn radiator hose. Also I have a complete radiator flush. About a week after getting all that work done, I checked the res tank and it was a little low so I simply added some more antifreeze just thinkin the radiator had sucked some up. I checked again a week later and it was low once again. I cannot see any visible fluid leaks externally, does this mean that the leak is internal, possibly a head gasket leak?

EDIT: I FOUND 2 LEAKS OF ANTIFREEZE ON GROUND, 1 BELOW RES TANK, and 1 BELOW RAD. CAP.

A just for reference when adding antifreeze/water mixture is it best to do it when the engine is off and cool, or engine off and warm, or engine running?

 

Tons of things , head gasket leak , cracked head, leaking freeze plugs ...
pop the radiator cap and start up the car and look for bubbles/smoke in the fluid that denotes a head gasket/cracked head