Period for a few hours

I’ve had an extremely regular cycle on and off birth control. My cycle is exactly 28 days and my ovulation has been day 16-18. On day 28 of my cycle I got a very brief “period” lasting only a few hours. It did not start/stop the way it normally does. My periods have always been light but lasting 3-5 days. On that morning I wiped and had bright red blood (usually starts as dark brown discharge). I used one tampon which it barely filled and then my period completely disappeared. I had no more discharge or bleeding. Prior to this 8dpo I had extremely sore breasts which I’ve never had as a period symptom. After this “period” I’ve been nauseous everyday all day long and still achy sore breasts. I’ve taken several hpt and they are all negative. I’m now 19dpo and not sure if I log this as a one day period or if it is implantation bleeding. I feel different this month and I’m not symptom spotting. Help! I feel crazy for thinking I can still be pregnant...