Why would an abuse investigator be hostile toward the reporter?

I am a mandated reporter and was told by a non-verbal special needs child's sister that the girl is sometimes restrained in what seemed to be an inappropriate way for the severity of the girl's handicap. I was uncertain of the exact circumstance so I called it in. When the report was made and the investigator called me back, she was very hostile towards me like I had done something wrong and was essentially accusing me of being frivolous with my report. The reason I made the report was because I thought that it was excessive restraint for this child based on experience and the investigator made it difficult even for me to express that simple fact. I was thinking about calling her supervisor because she made it so difficult to explain my reasons for concern. Does that seem like an appropriate action in this situation? Please be detailed in your response.