Most of the time, having sex after your period is unlikely to cause pregnancy because the process of the menstrual cycle is to get rid of the unfertilized egg, therefore, there is usually no egg to be fertilized after your period. But it is not impossible. Also, taking a test 2 days after you have had sex is not going to be accurate because you can get a false negative, but you can't get a false positive. A false neg is caused by not enough hormones accumulating during the time you have been pregnant. So if you were to be pregnant, it would not yet be detected.
You would have to wait at least 2 weeks, or after your next week that your period was supposed to occur (after your missed period, if it was missed) Good luck!