It was summer vacation and my best friend get the sideline job at our house being a maid. And then My parents and I went to vacation in a resort for 1 week. Suddenly, when I came back home I just heard the news that my boyfriend have slept into our house with my best friend just the two of them.. without my permission..
And then after a week my best friend just left in our house because of the reason she can't take it anymore.. is that because me and my boyfriend seeing each other that's why she is very jealous about and that was the reason she stop working with us.. please help...

There's only one thing that you have to ask if there was something happened between them both when you were away. If yes, then she have a feeling for your boyfriend.

Agreed, you have to ask her. We aren't psychic.
It's also possible that she just feels too weird being both your maid and your best friend. I think I might have a problem with it.
Best friends know everything about each other. I'm surprised that you haven't asked her.