Why are there bowel movements during final stages pregnancy and the causes of shortness of breathe even though iron supplements have been prescribed?

It is not unusual.: The uterus is so large that it pushes on the rectum and bladder causing frequency of urination and bowel movements. The shortness of breath, barring any pulmonary causes also is expected because the gravid uterus presses on the diaphragm.

Pressure: Theres is markedly increased abdominal pressure as the fetus descends. This also impacts the intraabominal space pushing on diaphragms.

Both normal: It's not that late pregnancy causes bowel movements: it's that when you "push" (to get the baby out), you push your bm too-so you want to be empty to avoid an accident. Shortness of breath is normal because your large uterus is interfering with the full expansion of your lungs. It will get all better in roughly 10 weeks!