The last time I had unprotected sex was on Dec 30, but I was also at the tail-end of my period. Unfortunately, and I'm not sure if this makes any difference or not, it was a Plan B-induced period (which I had taken in late Nov, and that resulted in an unusually heavy period that lasted for ~two weeks). Essentially, my cycle was messed up because of it.
I took Plan B (again) the following morning (Dec 31), which resulted in spotting/intermittent flows for a week or so, give or take. (And I know - it was unwise, Plan B shouldn't be taken in lieu of BC, etc. I'm coming to terms with that and shaking my head at myself for it.)
But now I woke up this morning to brown spotting. I'm concerned that it might be implantation bleeding, but because I've lost track of my cycle/regularity, I don't know where I'm at in my cycle and whether this is a possible sign of pregnancy or just old blood residue/normal.