hannah's mommy

So, my 5 year old just started kindergarten about a month ago. Within the month, she has had several different best friends already. It seems like she feels like you can only have one friend at a time, and if one of her friends is playing with someone else, they're not her friend that day, but the next day they're friends again. I can't keep up! And I don't get it - is this normal behavior, or should I be worried that she doesn't understand the nature of friendship? Or is this just how she is learning about it?