my menses just came this morning like after 2months after giving birth. May i know usually how long menses will last??

I gave birth last March 24 of this year and i had my period last June 4, it lasted only 3 days,. Since then it doesn’t arrive yet and im a little worried maybe i am pregnant again, but i did preg test and its negative,. I dont know why my period never came back,. Im “mix feeding” my baby of artificial milk and i breastfed also,. Can anybody tell me whats happening?,. Thanks