implantion bleeding and neg prego test

hey everyone, i need some help! My husband and I arent trying but if it happens great! I was on birth control pills for a long time but im bad at taking them, i was at the end of my prescription so i had to wait a whole month with no bc pills.Its been about 3 weeks since my last period, I woke up one morning and had dark blood on my undies, then had light bleeding all day and that night. Woke up the next morning and it was done. I saw that you get it 8 days after conception and the math adds up great since we could have conceived during the pull out method 8 days prior. i bought a test and it said negative. my period is normally regular but i dont keep track of it ever. so i guess ill just wait a couple days and test again. HELP!