AF after BC

So i started birth control on 7/21/17 stopped 7/28/17 has AF on 7/24/17 lasted 3 days was light, then 2 weeks later had light brown spotting for 3 days and havent had an AF since, my question is how long after BC do you get AF back? Even if i only took a weeks worth? Fyi im also getting bfns so im not pregnant just body is now screwy