Chances are the battery circuits need to be re-calibrated.
Run your battery down until your phone shuts off (do this just before bed), leave it off, and allow it to charge overnight. Power it back on in the morning and it SHOULD be back to normal.
Do NOT run your battery down until it dies on a regular basis. This is extremely hard on the battery and will shorten its lifespan. If done occasionally (less then once a month) it's fine but if you do it regularly you will eventually have to replace the battery.
If you notice your battery is already not lasting as long as it used to and you HAVE been running it down until it dies, buy a replacement. Buy a replacement that is directly from Samsung or a reputable manufacturer. Do not just buy the cheapest one off eBay as these are poor quality