Sophia, your symptoms don't sound like MC (we can experience some pain from tearing muscles when we're very stiff, but not joint pain), so probably it's another condition like fibromyalgia. I would suggest seeing a rheumatologist since they are best at diagnosing those types of disorders. Since it's affecting your shoulder, it's probably not a neurological disease.
Myotonia usually shows up from a few years of age to puberty. For the recessive type, Becker's, both parents would have to be carriers. If that's the type your dad has, you could have inherited one mutation, but not two unless your mother happened to be a carrier (VERY unlikely).
Your dad should be able to get DNA testing through one of the MDA clinics in your area. That way you would know the exact type and how it is inherited. EMGs can tell you whether you have myotonia, but not the type. There is no need to do muscle biopsies any more unless a doctor also suspects a mitochondrial disease in addition to the MC.
I have the recessive form of MC, with which it is not uncommon to have upper body weakness. Upper body weakness is a problem for me, and I do get pain in those muscles when cleaning, because my stamina in them is so poor and they will cramp from fatigue. I agree with Jan though, that what you describe sounds like it could be something other than MC, and you should see a doctor about it.