Sounds like my daughter (now 9). I had countless people tell me she had to be autistic even because of it. Even now my mom thinks something is "off". I disagree and so do her teachers, although in preschool they did worry because while she would play with others, she would not go to them to do so, she would play alone or let others come play with her.
At 9 she is independent, well spoken, stands up to her friends when she disagrees. Has a ton of friends, and is well liked. Does well in school, and gets along with everyone. Is she more serious than others, sure, she isn't the class clown but she also sneaks books when the class has to put their heads down, and gets in trouble for talking when she shouldn't sometimes. All in all a "normal" little girl.
So if you are worried, talk to your doctor, and keep up on it once school starts and see what teachers say, but chances are everything is fine.