HPT are pretty accurate and based on your symptoms it doesn't sound like you are. :-( Your body might just be having a crazy cycle this month and it's lasting longer than usual. Call you doctor and set up an appointment to find out what's going on.
I's not likely. Sounds more like just a fluke in your cycle. You can try tracking your BBT to see if you are ovulating. I went 64 days without a period a couple years ago, I went to my doc and that's the first thing he told me to do.
Call your doc and set up an appt. I was 11 days late and if I was still testing negative and no AF by the end of this week, my doc wanted me in. Better to play it safe!