How did I went from playing against high diamond-master tier players to playing against silvers?

Hello everyone, I want to ask you a few questions, how often does this happen and does this happen to someone at all?

First off all, I've always used to main rengar, I used to play him almost in every game, because I was unable to play other champions without degrading my mmr, also I was playing normals, mostly. I started playing in season 4 and I wasn't a good player, I was in silver and I would get matched against unranked players in normals, mostly.

In season 5, I started watching trick2g to learn how to play. After watching him a good amount of time, I started wining more and more games, to the point where I would get matched against high diamond-master tier players in normals. Than I decided to try ranked. I climbed from silver to plat 1 within days, getting 28 lp as an average with 70% win ratio on rengar. I stopped playing ranked.

In season 6, I had the same win ratio in ranked as rengar, but than I took a break for a few months, than I came back, suddenly I started losing most of my games and my win ratio dropped to 63 % and I decited to stop playing ranked. I went on playing normals only, but pretty soon I realised that I'm playing against plat players, most of the time, than gold and now silver players.

I used to get honored as Rengar almost every game for good plays and great kdas, but now it's season 7 and I can't do the same stuff I was doing before, I can't make those killing sprees anymore even against silver players. Back in season 5 I was hoping I could start maybe even streaming.

Heres my acc: http://www.lolking.net/summoner/eune/47049926/Andrionar#/ranked-stats

Edited*: I started degrading before rengar even got nerfed to the ground and it was before hes rewo