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.