Hi and welcome to the forum! I am glad that I can help you.
You want to know the cause of the symptoms that your mother experiences.
Fever and chills are symptoms that appear during infective diseases. These symptoms can be accompanied by other symptoms such as headache, stiffness of the neck (which may indicate meningitis), nausea, vomiting and diarrhea (which may indicate gastrointestinal infections), coughing and chest pain (which may indicate lung infection), myalgia and rash (which may indicate viral infection), etc. If these symptoms appear repeatedly on equal intervals, it may indicate infection with blood parasites such as malaria and others.
Given the data that you described, your mother experiences high fever and chills. She may or may not experience infection with blood parasites. She may consider seeing an infectologist for further diagnosis. She may need to be checked for possible parasites in a blood sample which should be taken in the moment when she experiences the fever.
Please keep in mind that I provide medical information only. I am not able to diagnose medical conditions online. Please advise your mother to contact her doctor or other qualified healthcare provider such as an infectologist for further advice and information about diagnosis and treatment options.