Aw, that's quite common actually, lalo na kapag breastfeeding pa, a few of us mums experienced super light or one-day periods months after giving birth, especially kapag frequent ang feeding like sa case mo na buong araw ang LO sa iyo. The brown spotting and then the light flow is actually normal as your body's adjusting pa lang, and since negative ang PT mo, it's likely just your period coming back slowly. That said, I'd still suggest checking in with your OB just to be safe, especially since it's your first one after delivery, walang nakakasakit sa peace of mind, di ba?