Have you asked your Doctor what he thinks , that may be a good idea, also have her ears checked make sure she is hearing you all . good luck
How is her eye contact - is it steady or does it just graze over your face? If you have a feeling something is wrong, then get pushy with the doctors to have her checked out thoroughly. It's not that big of a deal that she parallel plays and doesn't interact that much at this age, but it could mean something if other symptoms are present. It's possible she could be somewhere on the autistic spectrum. If this turns out to be the case, it's better to start therapy sooner than later. No one can say for sure what is going on...she needs to be checked out by a doctor. Hang in there. Maybe nothing is wrong...you need more input from doctors.
she needs to see a childs therapist