1996 Chevy Lumina Engine too hot, heater cold, loss of coola

I bought a 1996 Lumina "as is" from a dealer about 4 months ago. So far the car has run great, until a few days ago. The temp gage ran around the 230-240 degree mark & the heater started blowing ice-cold air. I called my usual mechanic who told me to check the coolant, which I did. The radiator was mostly full, so I topped that off with coolant (pre-mix), and the overflow tank was mostly empty so I filled that up, too. That was Tuesday night. Car ran fine all day Wed, but started the same thing thurs. Morning. Coolant in overflow was nearly gone. Filled it up again on my lunch break, drove about 16 miles, and when I checked it once I got back to work, the overflow tank was nearly emtpy again. BUT, I CANNOT find any apparent leaks, no dripping or pooling anywhere, no wet hoses, nothing. I should note that about 1 month ago, I turned on the heat after having heat off for a few days, and it made a sound like someone was pouring water on the passenger-side floor, but there was absolutely no moisture in the car and nothing dripping outside the car. It's done that a few times since. One more thing, when I added coolant to overflow tank today, I noticed there is some oily substance in the bottom of the tank. What could be going on here, and where is my coolant going? Please help, I'm a single mom with a daughter who just got her license & neither of us can afford to have this car die on us! And I'm very sorry that I cannot donate at this time, I do not own credit cards! Thanks!