As one mother to another mother I wouldn't be too concerned but I would watch her closely.  As a teacher, there are simply those kids that like to be alone and do what they want to do.  If she begins to distance herself completely from classmates and friends, begins to not participate during class instruction or discussions and spends absorbent amounts of time by herself in her room then you might consider seeking a professional to help you.  

Speak with her teacher next year and have him/her pay close attention to her peer interactions and her behavior in class.  Additionally, have the teacher note whether your daughter seems sad or argumentative toward her peers.  If such behaviors are manifesting it might be an indication there is a problem.  If you decide you would like some assistance in determining if there is an issue you might start with the school counselor.  Often times they can pull a student out of class for a few minutes.  Students often will easily trust and open up to a school counselor.  If nothing else the counselor should be able to provide you with resources to which you can turn for assistance.  Honestly, I wouldn't worry too much, but be watchful.  I noticed some students simply value personal time and don't want to be bothered with "entertaining" others.  

One other thought is there could've been a playground incident that you might not be aware of.  This could have her a little confused and uncertain how to move forward.  I watch alliances change all the time throughout the school year, especially between girls.  She sounds like a confident girl and she'll figure it out in her mind given some time.  Truly, it happens all the time!  I'm certainly no expert but I hope I helped a little.  :)