You seem like a good, caring Daddy to be even asking questions about your daughter's well being. My two cents is...to not sweat it. As long as she has met all her developmental milestones and you take her for regular pediatrician visits (Dr. would catch anything if it was a serious medical concern) than it is a matter of her comfort level.
You could try to get her around other children more often or maybe enroll in a class with her where she can play with others and still have you there. She'll come out of her shell when she's ready. She may just have a natural reserved, introverted nature when she's around others. As long as you know she's healthy than I would watch her body language (like facial expressions or turning away from someone) for signs of being comfortable or uncomfortable but I wouldn't worry if she doesn't want to talk to others, especially strangers.
Usually, if we parents make a big deal out of something then it can begin to make our kids feel that way too but if you act like it's no big deal then most things resolve themselves over time. You can still show her you love her whether she's got a quiet personality or whether she's a social bug and becomes Miss Chatty Cathy:) Take care!