I am 49. I have LGMD (exactly which of the 20 something LGMD types we don't know). My dad had LGMD too. He was the first in the family to ever have it. I have traced my family tree back to the 1700's and there's no mention of the illness in his family. Sometimes a bad gene can lie hidden for many generations and sometimes something can mutate the gene as the embryo is developing. Sometimes just a combination of the right (or wrong) two people can produce a child with MD.
Many people like my dad and others on this board find themselves saying "But nobody else in my family has it!" Unfortunately it just shows up sometimes.
MD is a genetic disease. You are born with a DNA mutation or deletion that causes it, either as a child or later in life. I've never in all my years of research heard of anyone "catching" MD or being exposed to a situation that "causes" MD. But I guess it's possible.
Addison disease, from what I know, is a problem with the adrenal gland. Muscle weakness is one of many symptoms. An EMG or muscle biopsy could possibly rule out MD. They would show muscle wasting, not just weakness. Talk to your doctor(s). Hopefully they could do some testing if they haven't already. Best of luck.