what is wrong with my boiler?

my caravan has a worcester combi boiler, it was only connected up yesterday & the water pressure gauge was set at between 1 & 1.5. i have not used the heating, but when i use any hot water the gauge rises to between 3&4 & the overflow runs/drips outside. what is likely to be causing this & is it a costly job to resolve. we have bled the radiators & the pressure dropped but as soon as we use the hot water again the gauge goes up quickly