Implantation bleeding?

So, I had sex on 9/3(7.4% chance of pregnancy) and 9/6(9% chance of pregnancy) 
​I ovulated on 9/11 
​Two nights ago on 9/17(8 days before AF is due) I peed and when I wiped there was very light pink discharge and I never saw it again. No cramping, nothing. I've never seen anything like it before and it looked different and was a different color than when I start spotting before AF.  I never spot more than 1-2 days before AF and I'm not due to start until 9/25. I'm never early, I start every 28 days like clockwork. I had a bad backache at work yesterday which isn't normal for me, and my boobs feel swollen and heavy and my nipples are all tingly. Also my nipples get dark during PMS and they're still light.. Any ideas? 
​Sorry for the novel ???