When you play your first game, the system does not know how good you are, so you start with a default MMR. Your earlier games are weighed more heavily in order to speed up the process of finding the best place for you, and as you play more games, they will have less and less of an impact on your MMR.
This means that your wins and losses early on will change your MMR significantly in both directions.
This is common for games with matchmaking, and usually helps in quickly getting you matched against players of your skill level.
This does not mean you are 'stuck' after you've played a few hundred games - it just means your MMR doesn't shift as drastically as it does in your first few games.