Pregnant again?!

My daughter is 3 months old and I am EBF.

On 9/6 I felt crampy and like I was going to get my period again but about a week later I only had very light spotting with light pink blood mostly only when I wiped and a little bit on a panty liner. There was one time where it was a little bit more and was a brown color. I took a test today (9/21) I think I MAY see a line, but my husband does not.

I also noticed from my Fitbit that my resting heart rate jumped about 15 bpm starting around the same time as the spotting..

How long after birth would you get a positive pregnancy test but it’s actually from the baby that was just born? Does anyone else see a faint line?