what does this mean???

i have a vauxhall zafira comfort 1.8 w reg 2000 model, i've been having problems with it overheating i sent it back to the garage where i purchased it cause i thought it was the head gasket, but when i picked it up he said it was the water pump which they changed free of charge, it have been running fine for the first couple of weeks but recently i have noticed that the temp has been going up and down even dropping to the first two lines of the temp gauge then rising again to normal temp, i have also had to top up the water in the car as it has dropped quite low. helpp!!!!!