Okay, there are way too many posts about ranked placements. So I'm going to explain in detail:
How these provisionals work is that when you start your provisional matches, Riot uses a formula to calculate the mmr at which you will start.
(Previous season's mmr + 1000)/2
You will ALWAYS start lower than your current rank. From there, if you lose any games, you will only go lower. Riot's logic is that if you truly belonged at your old rank, you will easily climb back to it.
Clearly since you didn't notice, master and challenger players dropped to plat 1 and will have to climb all the way back. Many plat players have dropped to low gold/high silver. So I'm sure you are fine and that you had a terrible mmr at the end of season 5.