How is MMR calculated?
Please be aware that your MMR is dependent on a baseline MMR determined by Riot.
There are a number of different factors that can affect the MMR of a given match, including the MMR of your teammates and your opponents as well.
Especially with the addition of Dynamic Queue, you can expect the MMR of each game to fluctuate a lot. So, please don’t think too much about MMR.
If you enjoy playing League you will get better MMR someday.