what is wrong?

After topping up our baxi boiler with water from the loop to bring it to the right gauge level which i do now and again I now find it keeps slowly going up on the gauge i keep bleeding the radiator but it continues to move up even over night when boiler is not on i also have checked the loop to see if it was turned off properly ,it was. There isnt any warning lights flashing and the heating and hot water work fine, this has never happened before. Any ideas