How possible is it for this to be implantation bleeding?

I'm 4 months postpartum and have had two periods since my c section. The first on July fourth, and the second on August fourth. The last time I had sex was Monday the 14th. Today, Monday the 21st - I had very light pink blood and now later tonight turned to a dark brown. Not really cramping - a little moody and slight headache. Never had implantation bleeding with my first.