5 days late! AF?

I thought I woke up today with my period because I saw pink when I wiped, so popped a tampon in and went about my day. It's been a few hours now and I went to the bathroom, took it out and there one one brown spot on it. Could I possibly be having implantation bleeding this late? I swore this cycle I'd be pregnant. No period, extreme nausea and fatigue. Thoughts? Is it too early to test if I'm just now having IB?