Vauxhall Vectra false reading with temperature gauge after working on exhaust ????

h, ive just bought a n reg vectra that needed the heat shield plate above the exhaust system re-fixed as it had worked loose, i refixed the shield onto the underside of the car but now the temperature gauge is going to red within 1-2 mins of when the car is started? if i drive the car onto a long straight road the temp gauge starts to drop after about 3-4 mins back to normal but when i stop it shoots back up again within 1-2 mins? the temp guage was fine when the exhaust shield was loose but i cant see how me fixing it back into its original position has caused the car to overheat? any ideas anyone ? kev