I wish people would research a bit before posting these threads... You are not the only one who is being placed lower than the rank they ended with last Season. In fact Riot stated this would be the case when the new season began, it was also like this last year and the year before that. This is what happens with a Soft Reset, you will be placed lower than the rank you ended with last season. I read in another post that they do something along the lines of taking your MMR from last season, adding 1200 to it, then dividing by 2. That's where you are before Placements Matches, and since these placements don't change your MMR too drastically you will end up either slightly above or below this new starting point (which is still below where you ended last season). If you truly deserve to be higher ranked, you will climb. If you are good you will climb fast, if not it will be a slow climb or perhaps a fall.
Good Luck.