It can be because of low blood sugar
or fall in blood pressure which are of course the commonest causes. But any other causes such as a possible seizure
without complete loss of consciousness
, transient ischaemic attack (where there is a temporary loss of blood supply to brain). Some of the rare causes can be of cardiac origin. Anemia also can cause weakness but it manifests mostly with shortness of breath
. So, please rule out the causes mentioned above.