What Causes Weakness,shaky Feeling In Leg And Palpitations?

my mum is 66 yrs old 4ft 10 gets 8 hrs with sleeping tablets and no pmh- her legs get weak and feel shaky inwardly. she constantly feels tired and started getting palpation's. the gp has put her on vitamin D which haven't hard any affect. do u have any ideas what's could be wrong ?