you might just have a naturally high tolerance. There are other factors like if you are taking it on a full or empty stomach and probably more that I am too lazy to write down. why is 1500mg (I'm assuming) of apap too much, does your body have problems with acetaminophen? if not you can take up to 4000mg In a day without harm, you probably would have problems with it if you did it everyday but as long as it's once or twice a week you are fine.
Also make sure they are actually hydrocodone pills, check the letters and numbers on both sides of the pill and type it into google. sometimes there can be mix ups like people using a vicodin pill bottle for other pills, dealers ripping you off, things like that, it's always a possibility.