TeenHelp - Implantation or period?

I'm back unfortunately. So today is the 21st, exactly 2 weeks after my missed period. My boyfriend and I had protected sex the 6th of October and my period came the next day but this month it never came. That is, until yesterday I started noticing that I was getting really moody and upset over nothing like I always do before my period. I've had light lower back pain as well for the past week. Today when I woke up my back pain was very severe as well as lower stomach pain so I figured I was getting my period. I took a pregnancy test last week and I just took one again this morning, both came up negative. Later on this afternoon I was going to the bathroom and had blood on my underwear so I put in a tampon. I had all the symptoms of my period, but I just took the tampon out and it wasn't even fully covered in blood I not bleeding anymore. I still have slight back pain though. Could this be implantation? Or stress or anything else?