So confused- what is going on with my body?

So, I am 27 years old, married and not actively TTC, but not quite preventing. I use this site to track my period, and as it turns out, I am 8 days late. I bought a 2 pack of the digital PT, took the first one wrong (dont even ask) and got back a negative result on the second one. That was on Wednesday (4 days ago) and I drink an obscene amount of water so it very well could have been diluted. Keeping this in mind, i just bought a 3 pack of non digital PTs and took one- came back negative again. All i had to drink today was a cup of coffee and maybe 4 oz of water - not really possible to be diluted. So here i am, 8 days late- which never ever happens- I'm usually very regular. I was pregnant once before and when I took the test it came back positive VERY quickly. I've been exercising on and off my whole life but just recently started mild weight lifting - could this throw off my period? I have NO symptoms of AF coming anytime soon... Is it possible to just skip a period? I wasn't really all that concerned, but now I am obsessing about knowing!