Kid fear - Less Social Interaction.

My daughter is 3 years and 10 months old. We are in Australia for last 2.5 years. She is an active child and reached all milestones correctly. She use to play with friends whom she feels comfortable, else she plays alone without mingling. Suddenly, for past one month she started to develop fear and cry even when she sees known people. She is taking at-least 30 mins to settle down and play. Whenever she is in other people home, after 1 hr she would ask us to leave and go back. She feels more comfortable in our home. Also she joined preschool 2 months back and do not see any problem there. Staff says she is having good time, but we see her play alone while we pick her back. She is learning to talk fluently and not talking to her age (She was diagnosed with basic level adenoid problem. Doctor said we can wait for another 2 years). Her social interaction is very less compared to kids of her age. We are literally worried about her behaviour.