Coolant leakage mystery

I HAVE been having a problem with the coolant system in my six-year-old Holden Vectra. At least every second day I have to top it up, otherwise I get a warning the coolant is low. When starting the engine it seems to shake for a minute or so before settling down. Three months ago I had it serviced and had the coolant flushed. When I took it back I was told there was a leak in the coolant system, but they suspected the head gasket might have a crack in it. Could there be a crack in the head gasket?