Brown spotting for 4+ days??

Hello all,

So I had protected sex 2 days before my last period, got my period as expected and thought nothing of it. Now I'm expecting another period in the next day or two, but have had this really light brown spotting for the past 4 days or so. Never had this before. So light that I can't even tell when I'm wearing dark underwear. Can only see small amounts of brown discharge when I wipe and today I wore a panty liner to get a better look, such a small small amount of light-medium colored brown discharge. Doesn't even really look like brown/old/dried blood. Has anyone else experienced this? Any thoughts? I don't think I could be pregnant since we used protection AND I got my scheduled period 2 days later as usual, but any input would be much appreciated. Thanks a million!!