It’s normal to be concerned about what’s happening and not to know what to say. It may help to come at this problem from a different angle. Sometimes it’s good to look at what’s behind a symptom. Your mother may find some meaning in putting up with pain without regular medication.
She may want to try to overcome the pain on her own, and this may be her way of trying to fight the disease that’s taking over her body. She may feel this is the only control she has right now. She may feel that taking pain medication means the cancer’s getting worse and she’s "giving up." She may worry about what medications will be used in the future if her pain worsens and she is already taking a strong pain medication now.
People can get defensive and shut down if they’re told to take pain medication without being educated about it. It may help to talk to your mother about why she’s refusing to use her pain medication. You may want to let her know how you feel about what you’re seeing, and ask her unthreatening questions. Consider words along these lines: "It appears to me that you’re in pain. It hurts me to see you this way and I’m really concerned about it. When you refuse to take regular medication, you have pain instead of relief. Why is it that you don’t want to take medication regularly? I’d like to know so I can try to understand what you’re going through. I love you very much and I want to see you comfortable. I’m wondering if I can do anything to help you." This approach may not work, but it’s worth a try. You may be surprised at your mother’s response.
It may help to talk to your mother’s health care team. It’s best to do this in her presence, as otherwise she may feel you’re interfering and going behind her back. If your mother is afraid of the medication and possible side effects, her heath care providers may have ways to minimize those. They also have information to help your mother better understand pain control.