It is anti-freeze/coolant. Anti-freeze is in the radiator, you radiator probably has a leak or your thermostat might be broken. Most likely it's the thermostat. They are like 10 bucks at the most to replace. The thermostat is on the top middle of the radiator, which is in the very front of the car. If your radiator is leaking, you could try to use some radiator sealers that they sell. My old truck leaked about half a gallon of coolant a day, I just started putting water in because it got so bad. I tried a sealer that costed 20 bucks and it fixed it so nothing leaked out (sorry I can't remember the name of it but I'm pretty sure I got it at a Wal-Mart). If those don't work then you'd have to get a new radiator. Those can be from $200 for used ones on older cars, but tend to cost more on newer cars. I'd go to an auto part store near you and get a thermostat for your car (need a specific one for your make and model). If you don't want to put the thermostat on the car then ask someone in the store to help you put it on, they usually will at a NAPA store, or have your boyfriend or dad do it, they should know how to put a new thermostat on since it is really easy. If it still leaks try one or two good sealers. And if it still leaks after that, then you need to take it to a mechanic. It might just be a loose hose, but you will probably need a new radiator if it you end up bringing it to a mechanic. Hoses may also have been loosened when they fixed the core heater since coolant runs through there too. If it is leaking under your seats in the middle, it means the hoses are probably lose, if it is leaking towards the front more, it means that it's the radiator. Good luck with all of that, hope I helped.