Has anyone else experienced this? Any answers would be amazing!?

I believe I experienced implantation bleeding about two weeks ago (which would've put me at about 10DPO (maybe less) - if I'm correct on my calculations) and I've been feeling weird ever since. Soon after I started experiencing mild headaches that came and went on their own (obviously I'm still experiencing these headaches) and now there's more: My don't hurt that much; they only hurt on the sides - towards my armpits - and my are a bit more perky than usual and my s also feel heavy. I've been feeling nauseous but not enough to actually vomit and as of about two days ago I've had cramps (only on my right side) and lower back pain. The back pain is was hurts the most! And as far as the "implantation bleeding" goes, it lasted less than one day, only once when I wiped. It was light pink in color and looked as though it was discharge. Then it stopped, I haven't had anything since. Well, I took a HPT 4 days ago and it came back negative (-) but I'm still having all of these pregnancy symptoms. I'm developing a headache as I type this! And the back pain has been consistent and won't go away. Is there a chance I may be pregnant? Did I test too early? My period was supposed to start yesterday but it hasn't come yet..