When the season ends your mmr also got partially reset towards whatever mean value riot decided to reset towards (traditionally 1200 elo under the old system). Towards the end of a season LP/mmr always gets inflated compared to the beginning. If you don't do well during those first 20 or so games especially if you don't win games the system had judged as favoring your team mmr wise to win, this can happen. Don't feel too down about it, you'll have to play but if your mmr is fine and you're getting matched against the type of players you say you are it'll correct itself since your lp gains are going to be big and you'll probably skip a division or 2 on the climb.