Why does my period keep coming early?

This is the 3rd week my period has come about 2 weeks after my last one. At first I thought my sister and I were syncing because I moved in with her recently, but it's the third time. I'm 15 and I don't have insurance, so I only want to go to a doctor if this is serious. I read that it could be stress, and I had to move from my parents house again and just started school last month, so could this be the cause?