Mine did this too. It jogged my memory that it happened with both of my other kids as well. My bleeding was down to brown spotting, and barely any at that, and then all of a sudden tere was bright red blood again. With my first I was on Depo and had no period after that bright red bleeding thing, except for some bleeding at three months that i never did know if it was a period or some residual something or what. WIth the second, I had the bright red stuff a few weeks post-partum, it went away, and I didn't bleed again until he was 13 months to the day. Here I sit, 3.5 weeks post-partum, bleeding bright red. A little disconcerting, but i know I've done it before and I don't believe it to be a period. I Beleive it to be due to activity level and exertion.
Just a note: Even with constant nursing, you can get pregnant. I have more than enough friends with babies less than 14 mos apart that will gladly attest to that.