I would have the radiator and cooling system pressure checked (inexpensive) to learn if there are any internal leaks.
If you have an automatic transmission keep in mind the oil cooler for the transmission most likely is inside one of the two radiator expansion tanks. If it has a cooler like this - two cooling lines one in and one out have the radiator removed and pressure check the transmission cooler inside the radiator expansion tank.
Be prepared to invest in a new radiator if it leaks. The cooling system should also be flushed and cleaned with a new thermostat and pressure cap. Call as many auto parts stores for a radiator as you can. An OEM (origional equipment manufacturer) price will give you heart failure.
It sounds like you have a head gasket problem..check your engine oil for signs of water or a milky colour...check your rad with the engine running to see if there are air bubbles.. replacing a head gasket could be expensive..when your engine is running look for a white smoke{steam coming out the exhaust pipe...good luck
You need to stop driving the car and have it check out immediately. If water get into the motor, it can do major damage and may have to replace the motor too. It happen to me once, very costly !!!!