It is sometimes possible that the iron tablets you are having can themselves cause black stools. However if you have black colored blood in your stools then it is a different thing.
The unawareness of bowel evacuation does occur sometimes and is not of any concern.
It is also normal to have back pain
during this stage of preganancy due to the weight of the uterus
If your referring to your pregnant uterus as being hard, then that too is normal due to the fluid present in your uterus. If however, you develop any pain or vaginal discharge
then it is important to consult your obstetrician.