Sex the day after period ends (CD 5). 4 weeks later, w/ light bleeding. Breakthrough bleeding, implantation bleeding or period? Normal cycle 35-37days

Do pregnancy test. Doing a pregnancy test is the answer. Although you had sex just after period ended it is always possible. Your period then could have been off and you just ovulated. You probably at 4 weeks having another period it is important to perform a pregnancy test.