Implantation spotting. Pregnant?

My bf and I are NOT ttc but, I think I may be having a scare.. 
​  About a week ago my bf told me he thinks we should get plan b because he couldn't tell if he pulled out in time. I was about to buy it and he said nevermind he knows he did. 
​ Well, I was ovulating when that happened and now I'm about a week before AF and I've spotting, at first it was very light pink and now it's dark brown. It's been going on for 4 days now. Still not a period and a week before it's due ! Could that be implantation spotting, if my period doesn't come how early can I take a pregnancy test? Here's our calander.