1st period after stopping birth control?

Sydney

I stopped my birth control on 7/3 and had a heavy withdrawal bleed from 7/8 to 7/12. I assumed that I would have a normal 28 day cycle so I should’ve gotten my period on 8/5. My husband and I have been having sex frequently since my withdrawal bleed. I have taken a few pregnancy tests and they have all been BFN’s. I though I was getting my period because I had some dark brown spotting yesterday, but it has now stopped. How long does it take for your cycle to regulate after birth control? From what I have read this could be early signs of a period or implantation bleeding. If it is my period and it’s just old blood, is it normal that it would stop? If it’s implantation bleeding, when should I test? Any thoughts or advice would be much appreciated!