ALS is largely a non-genetic disease--by that I mean it doesn't typically run in families. so not having a family member with ALS doesn't really mean anything one way or the other
having said that, you don't have ALS. and you almost certainly never will.
your left arm feeling "tired" and "achy" is NOT ALS. twitching is not a determiner of ALS either. twitching is most often caused by stress/anxiety, muscle over use and nerves just getting nervy. As far as I am concerned twitching is a great big YAWN.
Don't jump on the ALS fear band wagon. You don't have it. No one else on this board has it either. Most hypos will fear this at one time or another all because it is SCARY, TERRIBLE, almost always fatal, and a disease that takes a while to diagnose--not because docs don't think something is wrong but because it is a disease dx'd after other things are excluded first.. Which makes this disease a FAVE among hypos.