[RNEMESiS42]

My birdcage light has been coming on and off every once and a while ever since I bought my car four years ago. Around last April I was driving my car when all of a sudden I saw steam coming up from the hood. I pull over and see that my radiator overflow tank is filling up. The temperature gauge in my car looked normal. I drove home and parked my car indefinitely, fearing a blown head gasket. I haven't driven my car since then and recently decided to test it for a BHG. I purchased the $45 block test kit form NAPA, and tried it out. I did it twice, and got negative results for exhaust gases in the coolant. Afterward I drove it about a half mile and back, and noticed when I parked that the same issue is occurring: coolant spilling out into the overflow tank.
What could be wrong? Thermostat or radiator busted? I have no other signs that would relate to a BHG.