Spotting Early in Pregnancy?

AF was due a week ago and my breasts have been sore for weeks, which is unusual for me. I've also experienced some light nausea. My cycles are pretty regular, varying just by a day or two, so I am hopeful! I ovulate late in my cycle, about 9-10 days before AF is due, so if I am pregnant, it's probably very early. I just started spotting yesterday and it was very light pink. Today it's reddish brown, but I have no cramping. I have tested for pregnancy twice so far, both negative. I've read spotting can be a sign of implantation. In the past, I think I experienced what I thought was a period when I was pregnant with my first child, but that was five years ago, so my memory is fuzzy. I am waiting 48 hours after spotting to test for pregnancy again because I've read that it can take 48 hours after implantation for hcg to build up to a detectable amount...or should I wait till the spotting has stopped to test again? Are you experiencing or have you experienced anything similar?