1998 Chevy Cavalier

My car has recently started leaking massive amounts of coolant however it isnt leaking externally (there is nothing under my car). I have to add coolant everyday because the reservior gets completely dry. Than the light goes on and my car begins to overheat. I've had this problem before except that the coolant was leaking all over the driveway. I brought it to the mechanic but they found no leaks. They only replaced the coolant cap. Does this sound like something serious. I really cant afford another bill for this car!