The genetic testing should give you some definitive answers. From your symptoms, if it is indeed a true myotonia, and I had to guess, I would bet a chloride channel myotonia such as Thomsen or Becker MC, or a sub type called Myotonia Levior...you don't hear much about that last one but it's a very mild, dominant form of chloride channel myotonia congenita.
If you had a chloride channel myotonia, it would usually show up on Athena Diagnostic's test 128, the "CLCN1 DNA Sequencing Test"
If your insurance will pay for it, or you can afford it, I would ask the doctor to test for all forms of myotonia by doing the "Complete Myotonia Evaluation", test, 147, because it can save a lot of time and trouble, and some people have been found to have two different myotonia causing disorders.
As for the other symptoms, it's possible to have myotonia and an unrelated autoimmune disorder, as autoimmune disorders tend to be fairly common.
I suffer from the Sjögren Syndrom and Myotonia. My ANA is 1:350 and my eyes are absolutely dry as well as my throat and lungs. I do have stiffness, musclepain, fasciculations and symptoms of a mild periodic paralysis after activities. My EMG shows myotonic (or pseudomyotonic patterns, every doctor says something else). The muscular problems get worse with cold, hunger, stress, infections, menstruation/hormons and injuries.
My bloodresults are all ok, exept the mild increase of ANA, I dont have polymyositis, so it's not prouved that the sicca syndrom has an autoimmune cause. Many family members suffer/ed from the same problems, so it has to be a genetic defect. But up to now they didn't find the mutation, I should do a chromosomal mirco-array, but the costs are not covered.