Got re-placed after being placed already in 5v5 flex que.

I had completed my 10 placements games the other day in Flex 5v5 and got placed in Bronze 2 after winning 7 of my 10 matches. I know, not that good. Anyways, I then made it to promos and failed to complete it so I dropped down to Bronze 2 with 70 something LP. I stopped playing for 2 days since I had midterms and decided to play a match before bed. I went Morgana support (my main) and went 8/4/17 with 34 CS and a victory at 35min. At the end of the game it told me that "my journey this season starts in Bronze 4". How is it that I got placed in Bronze 2, and then after winning a game again (and doing pretty good), got placed in Bronze 4 after I had already been placed in Bronze 2? Can someone explain this to me? I know bronze isn't the best elo but it is quite annoying that I was at the top of Bronze 2, now I am at the bottom of Bronze 4.[zombie-brand-mindblown]