Hi Daisy,
Real sorry to hear about your mum and what she is going through right now with the treatments and the pain. The pain is definitely something that should be addressed and if your mum is having to use energy to fight the pain then that will just add to the fatigue that she is going through right now. The fatigue though with CC is very common and my dad went through that a lot, as have many and their loved ones here as well.
I don’t have anything specifically related to radiation pain but here are some links on cancer pain that I hope will be of use to you and your mum.
http://www.cancerresearchuk.org/about-cancer/coping-with-cancer/coping-physically/pain/types-and-causes-of-cancer-pain
http://www.macmillan.org.uk/information-and-support/coping/side-effects-and-symptoms/pain
http://www.cancer.org/treatment/treatmentsandsideeffects/physicalsideeffects/pain/pain-landing
http://www.mayoclinic.org/diseases-conditions/cancer/in-depth/cancer-pain/art-20045118
If I was you then I would go back to your mums doctor or onc and tell them that your mum is not coping well at all with the level of pain and see if they can change her meds or add to them. We also have a pain board here on the site so maybe you will find others experiences with pain in there.
My best to you and your mum,
Gavin