The reason could be, you never put in enough after you serviced it, or you created a leak during the service, either at the filter or maybe the sump plug. If all was ok before,and not consuming daft amounts of oil, these are the most likely causes as its not suddenly going to start burning oil. Ive assumed based on your post that you did the servicing yourself.
so as a first step, id check for leaks, signs of oil on thedrive etc and fix any. Then top up to the proper level, run the car as normal, and check level regularly to see if its dropping.