Can stage III colorectal cancer spread to the bones in two months?

Sadly my mum passed away from colorectal cancer. My mum was an independent 87 year old, she was diagnosed with stage III (cT3cN1M0) superior colorectal cancer in October. It was an invasive adenocarcinoma, with an enlargement of the rectum wall.

They did all the tests (including PET scan in December) and said there was no metastasis. Oncologist decided to treat it as palliative intention with low dose of radiotherapy treatment for four weeks. 

By the 7th session she started with low back pain which to begin with the radiotherapist said it was an isolated back ache not related to the radiotherapy, but the pain wasn't going away and medications were not strong enough to relieve the pain. After two weeks of no improvement the radiotherapist cancels the sessions because she had to be reassessed by the oncologist. We went to the oncologist appointment, my mum in a wheelchair (within two weeks she became totally dependent) begging the doctor to take her pain away.

The doctor refers her to palliative care, we asked all the questions whether radiotherapy treatment triggered the pain, he said no, and if it there was a possibility that she had metastasis, he said perhaps but they were not going to do any tests, his priority was to relieve the pain. So she was referred to palliative care team.

The palliative care team started her on Palexia (tapentadol) two a day one in the morning and one in the evening. Withing a week she had her dose of Palexia increased to three in the morning and three in the evening, as well as a one corticosteroid, three metamizol (analgesic) three paracetamol 1 gr plus all her regular heart medication and diuretics. She was a little better but not a great deal we were positive that my mum would improve a little.

At the end of the week she started having neck ache and was agitated again with pain. I waited a couple of days and called palliative care to come to the house (until then they were giving the prescriptions over the phone) to assess her pain I wasn't sure if my mum was just having zero pain tolerance or she was getting worse. I explained everything to them, he touched her back and she was screaming with pain but if she didn't move there was no much pain. The nurse then put a pump which the doctor said it was the same medication she was taking but in the pump so they can adjust what is the best dose for her and give it in tablet form. That first night that she had the pump she asked for a tablet in the middle of the night and we gave her what the doctor told us in case she had pain again.

Next day the nurse came back, puts more medication and gave us an antiinflammatory just in case she needed pain relieve later on. That same day in the afternoon she seemed a bit confused and very sleepy which they put down to the medication, I though she was now resting because she had no pain after all she had a lots of sleepless nights and agitation her body and mind was exhausted. I decided to call palliative care and they told me to try and wake her up other wise she would not sleep at night but I couldn't, when I was calling her she was opening her eyes and closing them again, sometimes telling me off for trying to wake her up. She had a nutritional drink in the evening went to bed and after three hours sleep she woke up complaining of shortness of breath, emergency doctor was called. A couple of minutes after they arrived they were saying that these symptoms are normal because of the drugs, as they were saying that my mum drops her head and dies. They tried to reanimate her but couldn't and finally pronounced her dead, heart failure. 

My doubts are if we didn't give radiotherapy perhaps she would still be alive, my sisters and I are so upset over this we feel betrayed by the doctors, we put all our trust in them and follow their advice and my mum started deteriorating since the radiotherapy. As well how comes she didn't have metastasis in December and the PET scan was done on the 28th of December 2016 and she has metastasis of the bones (not by test evidence but by symptoms) by the 10th of February as soon as she starts radiotherapy treatment?

Her oncologist didn't want to do any more tests so we never knew if she had metastasis, I think he was wrong not wanting to do an MRI, his priotity was to deal with the pain, but if we know what stage the cancer was we would have understood better the situation.

The doctors say it's nothing to do with the radiotherapy, but all I know is that my mum was well before treatment, has someone gone through the same or similar experience.

I would like to hear your opinions on the subject to put my mind at rest.

Thank you for reading my post.