Hey ae, 

Rectal peritonitis: the lining of the rectum is inflamed. Sometimes this can lead to sepsis, which can be fatal 

Perforated caecum: the caecum is a small pouch between the small and large intestine. With your Dad, it sounds like it had a hole in it. This means gastro fluids would have been leaking through, that's life threatening unless treated immediately. This could be the reason his rectal lining got inflamed. 
 

Rectal Carcinoma: cancer of the rectum. Surgery to remove cancer here is difficult because the rectum is a tight area of the lower colon. 
 

Your Dad had quite a few serious issues going on at the same time. You don't say what the doctors did for him or what they said his chances were of surviving. I'm surprised they didn't offer surgery, but maybe as I said it was difficult to operate on the rectum. 
 

Bowel cancer is a silent cancer, meaning people have no clue until it's in the last stage, usually they start feeling sick, vomit, have abdominal pain and then go see the doctor. Then they're shocked to find out it's last stage bowel cancer! That's why it's SO important to have a colonoscopy every 5yrs after you turn 50yrs old. And to eat a diet with fibre, vegetables and whole grains. 
 

Im so sorry for your loss. I lost a family member recently to lung cancer. They gave him 3 months, he managed only 1 month. Lung cancer can be silent too, and it affects non-smokers just as much. 
 

My guess would be, since I used to be a doctor, that your Dad would have felt abdominal pain, maybe had nausea, poor appetite, felt tired. It would have been difficult for him to go to the loo. Usually morphine is given for pain. If your Dad was on paracetamol then he would have felt some pain, I'm sorry. But he was a strong man. He tried to fight but sadly sometimes the body gives up or it's at too late a stage. 
 

All you can do now is grieve, go through the process of mourning. Be easy on yourself. If your Dad was still here, he'd be going through weeks of chemotherapy and radiotherapy and it would be a painful, difficult journey, his quality of life would have been poor. All I can say is that he was spared that ordeal. He's no longer in pain, he's at peace. 
 

My message to readers would be PLEASE get regular colonoscopies, eat healthy, exercise and live the best life you can. We all die but we can definitely help ourselves not get cancers or heart disease. 
 

Time will heal your pain. I promise you that. 
Take care.