the e-pills are effective for the first 24 hours with the success rate of more than 92%.however it loses its efficacy for the next 24 hours almost by 50% and so on which means that its almost ineffective on the 3rd and 4th day. the test you got negative might be the urine test for pregnancy
because it gives positive results after the 2 weeks of the conception. before 2 weeks of conception it will negative results. to confirm wether you are pregnant or not. you must do a blood test
named Beta HCG
which gives positive result from the first day of pregnancy.