Sudden Onset Of Nausea From A Long Time, Triggers On Moving Arms Or Bending, Normal Scans, Blood Work. Possible Causes?

My 83 year old Mother has had for several years sudden nausea that happens for no apparent reason. She has had scans, procedures, & blood work to check for ulcers , acid reflux , stomach emptying problems, heart conditions etc. with no major problems found. Her gall bladder has been removed for at least 10 years, as well as her female organs. She does have diviticulitis and high blood pressure, which she takes meds for. It seems that her nausea comes on rapidly when she is shopping or moving her arms or bending. Her gastro , family and heart doctors have not found any reasons for these nausea attacks. Can you offer any possible reasons for what is happening to her. She does not feel dizzy, have headaches or notice anything before these episodes happen.