Lump in the cheek. Can be the reason?

Hello. Hope you are doing well. The lump like thing which you mother has felt is mostly  a facial lymph node. They usually remain non-palpable (meaning you won't be able to feel them by hands) but they might grow in size and become palpable due to many reasons, most common being any infection in the facial region or oral cavity (including dental infections). Since she has been feeling the lump since childhood, that means she might have had on and off infections (including recurrent cold, tonsillitis, etc). Complaint of pain in such cases usually suggest an active infection which makes those lymph nodes more enlarged and little painful. It would be best to get her treated for any infection that she might be having currently, if any. If there aren't any possible causes then it would be better to visit and Oral and Maxillofacial Surgeon to get a thorough check up done.