How much blood is too much blood?

MBush4

I don't mean to be a worrier, but this is my first pregnancy and I'm paranoid about a miscarriage....

I just finished a walk around the park across the street from work. As usual, I had to use the bathroom after walking for 25 minutes (the joys of a pregnant bladder!). When I finished I noticed there was a little spotting. Not enough to turn the TP red, just light pink. And there was nothing showing in the toilet itself. At 8 weeks, should I be concerned and call my doctor? It seems a bit late for implantation bleeding since I've already seen my doc and had an US showing the baby was implanted. But it's not A LOT... any advice?