Couple of things.
1) It could just be a bad back. I hate shopping with my girlfriend, for the last 12 or so years I haven't been able to stand or walk about without getting a sore back after about an hour. If I'm active playing sport I don't notice it, but when bored and being dragged around the shops it really makes itself known.
2) There is a harmless condition called Benign Fasciculation Syndrome (BFS). This is where the body shakes/twitches/tremors. But it isn't linked and can't develop into anything else.
3) Pain is not associated with MND. And twitches would occur at the end of the muscle life. Also there is no sensation loss associated with MND. All you describe can be attributed to a trapped or pinched nerve.
4) The odds of you getting MND at 19 is ludicrously high, I'm not saying it doesn't happen, because like you, I have worried I had it numerous times in my life, and you read stories about young people getting it. But these people are an extreme rarity. The odds on you having MND are millions (even tens of millions) to one. You're more likely to be hit by lightning or win the lottery.
What you'll find is you pay more attention to symptoms that you didn't notice before. You will test muscle strength and try to see if there is weakness. By doing this though, you strain the muscles you aren't used to moving as much and then you notice it more.
I'll give you an example from my own history. I had numbness in my left big toe, and I felt I had muscle weakness too because of it (oh no MND). So I would walk on my tip toes all the time to see if I still could. Doing this over a period of weeks knackered out my heels and calf muscles because they were being used in different ways to what they were used to. So then I worried it had spread. I did it to myself. 4 years later, my big toe is still numb but I now know it is more likely to be nerve damage over my life that caused it. I am left footed and play a LOT of football. But that would be too obvious a solution