Hi and welcome
HUGS
I am very sorry about the cancer. It is much more likely that the cancer continues to spread through the body and is impacting on your piggy's mobility. Please make sure that she is on daily pain medication - her regained quality of life and her renewed zest of life will contribute more to a longer life span than any other concerns. I have had several arthritic oldies of mine on long term higher dosage dog strength (1.5 mg/ml) metacam (meloxicam) so they could enjoy life for over a year longer without any of them ever developing kidney problems.
Sadly, cancer in guinea pigs has become a lot more common in recent years but it is generally not treatable unless it is a still encapsulated small lump that is easy to remove. Once it has started seeding around and spreading through the body, there is nothing you and your vet can do apart from pain relief. Guinea pigs have a much faster metabolism than humans, so it all happens on fast forward.
The fact that your piggy has reached a good normal life span is testament to your good care, and your love is shining clearly through. We can sadly never choose what happens in terms of health and in which way the end comes beyond our normal good care.
You may find these very useful guide links below helpful. They contain lots of very practical tips on how you can make your piggy's life more comfortable and adapt to their developing needs. Even little changes can make a big impact.
The advice in this link will help you with the increasing mobility issues, including easy access to food and especially water, potential follow-on problems and what you can do in terms of enrichment to still bring little joys into your piggy's life.
Looking After Guinea Pigs With Limited or No Mobility
Health monitoring explained: what the various methods do and don't and when you need to step in with the appropriate level of feeding support.
Weight and Weight Loss Explained: BMI, Weighing, Poos and Feeding Support
All About Syringe Feeding and Medicating Guinea Pigs with Videos and Pictures
These guides below are for the case that your beloved little one is suddenly taking a turn for the worst. I am never happy to link them into a thread but since you are in a considerably different time zone to where our forum is based, it means that you have at least easy access to any necessary information so you are hopefully not going to feel so badly overwhelmed and helpless when you are in a panic.
Emergency and Crisis Care as well as Bridging Care until a Vet Appointment
A Practical and Sensitive Guide to Dying, Terminal Illness and Euthanasia in Guinea Pigs
We have also got a dedicated End of Life and Bereavement section which offers ongoing community support to owners for all the little and large questions and the huge and not necessarily expected emotions that will arise along the way.
My thoughts are with you in this difficult time.