2000 Honda Civic coolant problem

I have 2000 civic. About six months ago, my engine became warm not hot and the temp gauge would get up near hot then drop to normal. It did this several times before I stopped. I let the engine cool and found the coolant level in the radiator low. I decided to replace the coolant and thermostat flushing all old coolant out of system (heater core,head drained etc). It was fine for all this time until recently my level is low again and collant in recovery tank was high. I transferred the coolant back to radiator from tank manually making the recovery tank at the low level and the radiator almost full. I added more coolant to fill it full. After three days I checked the recovery and all is well, but the radiator was just 150cc low on coolant. No leaks are visable and no white smoke from exhaust. This has me puzzled as to what is going on???