SOO CONFUSED?😳🤔

So I took a first response digital pregnancy test as a haha situation to prove my sisters wrong...I just got off my period a few days ago..well it came back POSITIVE?!?! HOW??? It was a true 5 day AF with a medium/ heavy flow...I do have another clear blue digital pregnancy test and gonna buy a few more I guess to see if this is a faulty test..my sister in law gave it to me yesterday and literally just took this test, for more of a "told you soo"..but jokes on me I think..this will make baby #5 if it is a true positive..what do you ladies think? Never have I had a positive pregnancy test days after my AF so this is confusing. Got my Mireana removed on August 1st..sorry this is a long rambling post but I dont want to tell my Fiancee yet just incase...