Hello and thankyou for approaching DoctorSpring with your query.
I have gone through your mother's case and I understand your concern.
Her condition is nothing to worry about. It can probably be due a muscle pull due to a wrong sleeping position or due to folliculitis.
Folliculitis is an infection of the hair follicle. It is probably in the beginning stage and hence she has not felt any boil yet. The inflammation tends to be tighten the skin causing the pain.
I would advise you to give her the OTC tablet Ibuprofen 400mg SOS ( when required, if the pain is still present) on a full stomach. It will help reduce the pain and inflammation.
Since she has not felt any lump (lymphnode swelling) in the armpit or any lumps in the breast , it is nothing to worry about. It will be due to a local cause.
You can give her Ibuprofen till she visits the doctor.
I hope this has helped. Feel free to follow up with any query