It may not make a lot of sense, but you have to be willing to understand it. Most likely I'm assuming you were low Silver, or a Bronze 1/2 at the most. In which case almost everyone drops significantly when the season restarts. Plus 5 W / 5 L is not very spectacular as well, no offense.
Although I must admit, I find it funny that I went 4 W / 6 L in placements and got Bronze 4 lol. If you're good, you'll climb in no time. That's about all I've got to say. The more you ponder it, the more you blame yourself or the system, the longer it's gonna take to get to where you "think" you should be or where you "were" at.
1 game at a time brah, the struggle is real...