Clare, there is a form of myasthenia which is associated with myotonia. You would not have the typical MG antibodies if you have a congenital form (and it can have later onset). Congenital Myasthenic Syndromes
There is something called hypokalemic periodic paralysis which might be worth checking. It usually appears early in life, though, so your low level may have been a reaction. Are you taking any steroids? That can lower potassium.
Mitochondrial myopathies are very often associated with diaphragm weakness. They are usually inherited but may not appear until later in life. This is an overview...they are usually diagnosed by a neurologist specializing in metabolic disease (and usually they are only at children's hospitals): Myopathies