\\\\I'm a usually fit and healthy 36 year old until 4 weeks ago.\\\
That is against the diagnosis of Addison's Disease, which takes months or years of ill health before diagnosis'
Addison'e disease is extremely uncommon and the main symptom is an extreme desire for salt.....salt on chips, salt on steaks...... almost an obsession for salt.
You can Google the symptoms yourself.
The diagnostic feature is a low blood sodium level and this may well have been done i the kidney tests that you have had which cam back normal.
Ask you doctor if that will reassure you but the chances of you having this disorder is remote.