There may be several reasons for your mother's behavior. It's possible she is forgetting that she took her medications and repeats the dose unnecessarily. If she is taking other drugs, she may be confusing one for another. It may be helpful to use a pill dispenser to make sure she gets all her medications in the right dose and with the correct timing.
It's also possible that she is having breakthrough pain. Sometimes patients really do need a higher dose of a narcotic but they don't get it because of fears that they may become addicted.
Are you aware of any other drug seeking behaviors? Has your mother told the pharmacist she lost her pills and needs a new prescription? Is she going to more than one doctor with the same complaints of pain seeking extra pills?
If your mother will allow it, make an appointment with her doctor and go together. Perhaps a phone call before the appointment to alert the doctor of the pharmacist's report and your concerns would be helpful.
It may be she just needs a medication or dosage adjustment. There may be a simple solution if she is having breakthrough pain or she's being inadequately treated. If she has become dependent or even addicted to the drug, then another plan of care may be needed. The first step is to have her situation re-evaluated by her prescribing physician.