Spotting before period is due?

My husband and I have been trying to conceive baby number 3. My last period was on 21 September, and the next is due 19 October (my cycle is always exactly 28 days). However, today is 16 October, and I noticed I have had some very light pinkish spotting for the past 5 hours or so. It is not like a normal period, or even the start of one, and most of the time, I only see it when I wipe. The possibility of it being implantation bleeding crossed my mind, but isn't it too late in my cycle for that (since my period is due in 3 days)? It is very rare for me to spot between periods, but I did have some unexplained bleeding during my other two pregnancies (which turned out to be fine and I went on to have two healthy children), but they were at 8 weeks and 6 weeks. Any ideas?