You could totally be pregnant! The same thing happened with my sister-in-law. She gave birth in early June and conceived again in Sept. While it's not recommended by doctors to conceive again quickly after giving birth, she now has two beautiful healthy children that are the best of friends because they are so close in age. Keep us posted of your test(s).
If your not protecting your self then yes you probably are by the sounds of things sounds like you had implantation bleeding, are you late? I would test asap!
WELL.... if your not using some sort of BC then you obv have the chance of pregnancy.... regardless of whether or not your BFeeding. So i suggest you take a test to knwo for sure.