This is normal for your situation.
Ammonia in a pool is very VERY chlorine intensive to get rid of. As someone mentioned before it takes about 10 ppm Chlorine to get rid of 1 ppm Ammonia. So this behavior is to be expected. What will happen is a TON of chlorine will be used very quickly, and then just as quickly will stabilize out once you have put in enough to counteract all the PPM of ammonia that is in there.
For example, if you had 15 PPM of ammonia, it would take 150 (<-- see the zero on the end there) ppm of chlorine to get rid of it, and then even mroe chlorine to get rid of the neutralized ammonia that is now combined with the chlorine (CC). We dont know exactly how much ammonia you have so we are just telling you to keep feeding your ammonia monster chlorine on a frequent and regular basis (every hour, or couple of hours) until its dead. Then things will settle down dramatically.