it could be implantation or just a very light af I'm not 100% sure or the birth control might just be changing ur af every women is different you could always test best of luck
No but I'm on birth control I didn't start on cycle day one at all I was in the last week on my cycle and I had ovarian cysts and I had a super light period it seems like on cycle day 16 and it was like so very light 3 days almost literally the blood was turning brown the end of day and start with pinkish then brown by the end of day and I had to go inside to get it out