8 days late, brown spotting

Af was supposed to be here 8 days ago. August 8th I wiped and the toilet paper had dark brown discharge on it. I know that it's old blood and there wasn't enough to fill a pad or tampon. I had a little bit today but not enough to really notice. I woke up a little nauseous and it's off and on now, my lower back has been hurting (I've never had issues with it), I'm more tired than usual, I have gas (which I usually don't), and I feel bloated. could this be implantation bleeding or maybe just an off period? I took a hpt and it came up negative.