Period due, very little spotting

According to glow, my period was due on the 31st. I just started using it last month. According to my calculations though, I thought it was due the 3rd or 4th. Either way, when I get my period it may be light in the first few hours but for sure by the next day, it's a heavy normal flow. Yesterday the 31st, I had a few brown drops of blood when I wiped. Thought it was my period. Today the same thing. I out a tampon in thinking it was coming. And there was one drop on it. I've never really had this light of spotting before my period ever. I had light lower abdominal cramps for half of the day and now they are completely gone. So I tested and it was negative. Any idea of what is going on? I'm so confused.