My beloved dog was pts.

Last week my beautiful 15 year old dog was pts. I had noticed some changes with her for a few weeks but she was always active and eating and drinking. on the Monday she took to her bed and spent most of her days sleeping and had lost interest in everything. The day before she passed I had to carry her home from her walk as she stood still and couldn’t walk home. She’s never done that before as she loved her walks.
She had lumps on her belly for years and they were checked out by various vets and we were told to keep an eye on them.
on checking her belly we noticed one of her lumps was bleeding and she kept licking It. We took her to the vets and were told that the lump was cancerous and she was in extreme pain and we should put her sleep. We are distraught to say the least as apart from her fatigue and lack of appetite in her last few days she didn’t appear to be in any pain.
I know dogs show symptoms in other ways but I just feel that she declined in a matter of days and we weren’t prepared for her passing. Has this sudden decline ever happened to anybody else’s pet dog. I am struggling with her loss and how quickly her health deteriorated from a Monday to Thursday.