Danni,
Since you had a c/sec it's very unlikely that it's retained placenta (still possible, though). Before 6 weeks, it's almost certainly lochia. I've had 2 c/secs, my first & last deliveries. I had a "last gasp" of lochia at week 5/6 with both. With my first baby, like you I thought it was AF & I couldn't understand why, since he was feeding so well. The last time, I rang my doctor. He told me to give it 48 hours & then come in to Delivery Suite if it hadn't lightened up. Sure enough it was lighter by then, & within 5 days that was the end of it. I just got my first post-birth AF earlier this month.
I read a book years ago called "The Ecology of Breastfeeding" (or something like that) which mentioned this phenomenon. It's the only place I've ever read about it, but the book described exactly what I've experienced & said that it does happen to some women, & specifically at about the 6 week mark.
Do ring your doctor within a few days if it's not lighter. HTH.