This is termed as Braekthroughl bleeding while still taking the pill. It is the commonest side effect for those especially taking the pill for 1-2 cycles. The tissue you saw was probably just endometrial tissue which is normal. You probably have a thicker inner uterine lining which is why there is breakthrough bleeding
while being on the pill. You may visit your Gynaecologist if the pain and bleeding do not subside with 2-3 days.