As Marc posted, .5 lbs to 1.5 lbs weight loss throughout the night is not alarming and pretty normal for a teenager or adult. It's pretty typical for mine to be upwards of 3 lbs of difference from night till morning. Weigh yourself at roughly the same time each day to get a more accurate result. That said, don't be too alarmed at small changes. Things like how much water you drink, what you eat and activity level all make your weight fluctuate. My doctor said he'd only be concerned if it were consistently dropping and I wasn't doing anything to cause it to drop.
Typically, weakness precedes atrophy with ALS. The reason your muscle atrophies is lack of use whether that be due to ALS or your arm being in a cast. I went through the same thing as you and was constantly comparing my hands to each other. One thing you have to remember is that your hands are naturally going to be different as one is dominant in terms of usage. You use one to open doors, twist caps, etc... much more than the other. I play guitar and so my hand has more defined muscles in relation to the fingers used for pressing down strings but my grip is stronger on my right because I use it so much more.
Did your doctor prescribe an SSRI (Lexapro, Celexa, etc...) or was it a Benzo (Xanax, Klonopin). SSRI's take many weeks to feel the effects and sometimes have side effects of making you more anxious during those weeks. My left hand would tremor if I tried to hold a business card between my forefinger and thumb. I had twitching all over but my left hand and right foot were the two areas I was most concerned about. Ultimately it takes time for your nervous system to heal. I was a wreck and multiple doctors just kept telling me this stuff doesn't turn off like a switch. But it is scary because it feels like your life is being threatened and it's really hard to just stopped being scared of that. The best advice I can give you is to stop googling and looking up stuff about ALS and be proactive in dealing with your anxiety. Sleep is super important as lack of sleep creates a vicious cycle of increased anxiety. Try things like meditation, yoga and exercise. Lastly, there are some great stories on here about people struggling with the same fears you are and all you have to do is type ALS in the search feature to find most of them. Hope this helps.