Any mommas have weird periods after giving birth and breast

Hey everyone. I had my first period after having my son on May 18th, and now I'm still waiting on my period to show up. I had some brown discharge on June 13th, but that has been it. I thought it was my period starting but apparently not :/ I am also breast feeding 90% of the time and supplementing with formula occasionally. Could this be implantation bleeding and possibly me pregnant again or is this just my cycles acting weird as hell?? I took a test to see but it said negative. Thanks for any input!