Long story short you probably got carried the first couple games and lost a bunch after that, and the initial wins bumps your MMR higher than it should be which makes the losses not hit your MMR as hard.
You basically just haven't played enough ranked to get demoted to your skill level. You're just playing against people that understand the game better than you because your placements more or less cheesed the system.
The most important thing you need to focus on at this point is trying your best to get more and more last hits on the minion waves. Don't hit them with auto attacks for no reason, you want to only tap them for the last hit of their life so you get the gold. There's a lot more to lane control than only last hitting, there's times when you want to push or thin the wave out etc., but you don't need to worry about that until you can say that you're basically getting 90% of the minions for the laning phase.
Just a heads up, don't get too upset when you demote out of Silver. Just focus on getting better and you'll eventually pass your old rank. If you need to, load up custom games every so often and just practice last hitting for 20 minutes. Try and improve your CS every time you do it. This is the single most important thing you need to actually practice because you basically aren't getting any money whatsoever looking at your ranked match history. Also while you're practicing try to get used to telling your champion to move right after they swing/shoot their autoattack. So you want to shoot, move, shoot, move, shoot, move. It'll be hard at first, but it's necessary to eventually being good at the game.
There's a lot of things you need to do to improve, but last hitting is the place for you to start right now.
