Advice please

So AF is 4 days late. My peak ovulation was on the 12th (checked using opk ) and we BD'd that day. Had light bleed on the 19th which I thought was implantation bleeding along with cramping. Next day was fine but have been feeling nauseous, sore nipples and breasts, back aches and intermittent cramping. Have taken 3 hpt's over the past 3 days and all negative. Can it still be too early for hcg to be detected? I thought 16 dpo would be plenty of time. Would love to know your thoughts or if anyone's had a similar experience. Thanks ☺