Thanks for the query.
Sometimes during menstruation
endometrial tissue will shed along with blood.
So the tissue you mentioned could be endometrial piece.
If the bleeding stops by time, no need to worry.
If the bleeding increases better to consult gynecologist once and get examined.
For more details you can ask me through: http://www.healthcaremagic.com/doctors/dr-sree-gouri-sr/63429