Driving licence LOST for 15 days after use of cocaine

Hi everyone.I have a big dilemma in what concerning my loose of driving license after 2-4 days of use cocaine.Saturday I have use coke ....Wensday I drive with my car and get stoped by the police.Thei request me to make a mouth test for drugs....Amazing but this turn up to be coke positive.The strange part is that i have used that 4 days before the test.....SO i lost my drive license for 15 days....It is really possible the test to be positive after 4 days of using the drug ???
Or can drugs tests give fals positive results ?? Any infotmation about this ..??? If yes please give me a reply with an explanation because really i got tottaly mad when thei told me thei take the drive license because i drive under the INVLOED VAN COCAINE.....