Does it smell and look like coolant? Cooalnt is usually greenish, but can be a different color depending what was filled in.
Coolant smells a bit sweet? But you can check and compare both liquids.
Do you have loss of coolant?
If coolant, you might have a leaking heater core. Just use the search with 'heater core' and you will find the info how to change that one.
If no loss of coolant, no smell of coolant, maybe some drains blocked.