I quit pot on 1/10/2013. i was tested for pot and it was negative. two days later it was positive. how is this possible?

Depends: This depends on the type of testing being done. There are false positives. That said, one of the most common causes of a positive test in a prior smoker is ongoing exposure to second hand smoke. If your friends/family smoke marijuana around you, you will be inhaling and test positive as if you put marijuana to your lips and smoked it directly.