What do you ladies think? Advice!!

My AF was supposed to be here on October 25. I am now 10 days late. I've been working out so I know that can delay but here's the catch. Yesterday I started some crazy heartburn and I NEVER had it before. We were active last night and this morning I went to the bathroom and had very light brown discharge on the TP .. I put a panty liner on and i have very little brown on it. I've noticed lower back pain and I deff get tired quicker but I'm a busy person.. Even though I know it wouldn't show I still tested with a big fat NEG. So now I wonder if this is implantation bleeding? What do yall think? Thanks ladies!