Implantation?

So I ovulated on the 22nd of Feb and had a fertile window of 5 days. 3 days before and 1 after the day of ovulation. Me and my partner had sex 3 days before I ovulated and I know I ovulated due to the opk. I got a positve opk the day before ovulation and had ovulation cramping that day and the day after. I was due on my period yesterday (9th march) and I've gotten some very light bleeding. I had it on the 8th, 9th and again today (10th). I use menstrual cups and there's barely anything in it. Literally CM that is a bit tinged brown TMI. Usually if it was my period I'd be very heavy yesterday and today. I also get really sore side of my boobs and nipples when I'm due on my period, I've had no pain though now which is strange. I've had some very light cramping also. A few days ago I also felt really nauseous but thought it might be PMS. Could it be implantation? I know some women say it lasted a few days for them but others said it lasted a few hours. I'm 16 DPO and the bleeding started at 14 DPO. I've taken a pregnancy test the day before I started bleeding and it was negative so.. any ideas?