Arm fatigue vs. weakness

I'm a 29-year-old male and I'm trying to figure out what I'm dealing with. ALS has been worrying me lately. Basically, my upper arm, just under my left shoulder, feels VERY tired after exertion. I don't think it's any weaker than usual, but if I do push-ups or carry something or even just keep my arm raised for an extended period, it just feels really tired. A huge difference from my right arm, which is fine. This has been going on for a few weeks and I don't think I injured it or anything. There are also fasciculations. I've noticed them in the same upper left upper arm area, but also elsewhere. They aren't very frequent, but lately they've also felt "deeper" (i.e. I can feel myself twitch, but it's probably not visible on the surface). I've read this could be a sign of ALS, as opposed to Benign Fasciculations. So I'm really starting to get nervous. I wake up in the morning and my arm usually feels OK, but after just walking around and being awake for a few hours, it can start to feel very tired. I wouldn't call it muscle cramping, but the muscles just seem tired and when I use them, I definitely feel it. Just wondering if anyone else has been through something like this. Thanks.