Hey I Had A Period On Feb 5th. It Done

Hey I had a period on Feb 5th. It done on the 11th it was to come then the 5th of march n it did not I then did 3 test to see if I am pregnant there where neg I had spotting from the 15 of march till now I went to the doc on march 21st n had to done a test there n it was pos I went and get a scan done and the doc say there is a sack there but y I am spotting so long if I am pregnant.