Hi and welcome to the forum! I am glad that I can help you.
You would like to know the cause of nausea and vomiting.
Nausea and vomiting may appear due to: gastrointestinal infections, acid reflux or increased cerebrospinal pressure due to meningitis or tumors. Vomiting may cause a fluid and electrolyte unbalance that may lead to dehydration and cardiac shock.
According to the data that you described, your mother may experience vomiting due to an underlying gastrointestinal infection. However, given the data of her age, she may be at risk of experiencing vomiting due to increased cereblospinal pressure due to a tumor. Brain tumors may not always be malignant. They can be benign, but due to their location they may act malignantly by pressing over other brain structures. She may consider having CT scan on her head to rule out a brain tumor. She may also consider having a spinal puncture and having bacterial culture and laboratory analysis of its sample to rule out or confirm meningitis.
Please keep in mind that I provide medical information only. I am not able to diagnose medical conditions online. Please contact your doctor for further advice and information about diagnosis and treatment of your mother's current condition.