Temperature gauge on Citroen C3 (1.1) keeps spiking up ?

It spikes up and gives a warning beep then goes back down to its regular temperature reading (i.e. 3 bars) approx 20 minutes into a journey . It was slightly racing upon starting up a few weeks back too. However the car seems to be running ok...any ideas what the temperature beep is warning me about as it isn't overheating?? Only had it serviced a couple months back too.
K