You are experiencing the classic symptom of a leaking heater core. They aren't cheap. Some stop-leak in the radiator might get you through the interim if you need to budget it.
Antifreeze leaking from the heater core.
Passenger side floor? Could be leaking heater core
Leaking heater core.
Heater core leaking?
Sounds like heater core is leaking and what you are seeing is antifreeze
Heater core has failed.
You have a leaking heater core. You might smell antifreeze if you turn on the defroster.
Leaking heater core
It sounds like the heater core is leaking. Check for the smell of antifreeze and that will confirm the bad heater core.
Its probably the antifreeze in your cooling system. There is a good chance the heater core is leaking. Check the front passenger side floor to see if it is wet.
Sounds like your heater core is leaking. Does the fluid look/smell like antifreeze? if so, time for a new heater core.
Usually, you'll get antifreeze on your passenger floor, steam out the defrost and / or have a very noticeable sweet coolant smell in the car. The heat won't work well and the car will overheat.