You should have left and called the police. Also, you should consider not going there alone.

Have someone go with you, with a video camera, if possible. They could go in a separate car and monitor without your ex knowing.

You might also consider a neutral drop off point for future exchanges, to avoid these type of problems. Most important, you should document all contact with her, and any lovers, for future reference if it goes to court.

Keep a journal, carry a micro recorder, and/or have witnesses.