There are a few possibilities. It could be that the implant came later. That would be the good news.
However you could be facing what is called a missed miscarriage. You do have an implant but it isn't a healthy one so the hormones rise slowly. This happened to me with my second pregnancy. HPT showed pregnant but hormone levels showed such slow growth I was warned that things didn't look good. My first u/s showed a gestational sack but no heartbeat and no sign of a baby. I ended up having a d and c at about 8 weeks.
I hope this isn't the case for you, but you should be prepared for the possibility that this cycle might not be successful.
On the up side, it is always possible that your implant was a little later than usual and things will turn out ok. Try not to do any more HPTs as they will just get you wound up. I'm sorry you have to deal with such uncertainty. I'll keep my fingers crossed for you.