Salmankaleem,
An altimeter is nothing more than a device for measuring air pressure. The altimeter has no idea how high it really is; it only knows about air pressure. It's designed on the premise that air pressure decreases at a known rate, with altitude. If the altimeter sits on the ground and air pressure changes as weather or a front moves through, the altimeter reading will change...because it's sensing air pressure.
A difference of a thousand feet is possible on an altimeter with the passage of a front, though it's seldom that great. Several hundred feet isn't at all uncommon. To correct the altimeter reading, check the local altimeter setting and put it in the altimeter window ("Kollsman window"), or simply adjust the altimeter until it indicates field elevation.