Hi. Thanks for asking in Healthcare Magic. The main mechanism by which an hormonal injection acts is through suppression of ovulation. An ovulatory cycles are painless and that s why your Dr has said so. Generally it takes some time for the hormones to act as they are administered externally and the body takes some time to get adjusted to this hormonal imbalance. So, they may not be effective in preventing the ovulation in the very first month. If the ovulation has occurred already, then hormone injection will not prevent the fertilization but will certainly prevent the implantation. The tissue you saw could have been the expelled fertilized ovum. Also heavy bleeding and delayed periods are common in the initial few months. If your period had been a little delayed following the hormone shot, then the tissue could be endometrium which is thickened when there is delayed period. If the bleeding persists and if you pass a lot of clots, then you should seek your treating Dr s opinion. Otherwise, the heavy bleeding may be because of the hormonal imbalance due to the injection. I hope this is useful to you.