Thanks for the query.
Even being exclusively breast feeding some women can recurrence of menstrual cycle early.
As you didn't mention any pain abdomen, foul smelling vaginal discharge
etc. there is less possibility for infection or retained products.
So this could be due to hormonal
fluctuation and the bleeding will subside soon.
If the bleeding continues better to consult gynecologist once and undergo ultrasound
to find out the possible cause and take treatment accordingly.