The teacher is required by law to report this information. I don't understand why she didn't ask your son more questions or talk with you before she called. This is upsetting, I know. I suggest that it may not have been the teacher. Another parent may have called based on what her child told her. Or any adult at the school; probably one who doesn't know you and who is a bit naive. It's very possible that the investigator didn't know who his teacher is specifically but knew the complaint came by way of the school, an adult employee or a parent who got the information from what your son said at the school.
I suggest that you talk with the teacher and find out what happened from her viewpoint. Don't accuse her of calling. She may very well have not called. I suggest that if you're calm and have an open mind she may be able to answer your questions in a manner helpful to you. If you can express your embarrassment/being upset in a non-threatening way this can enable you to have better communication with her and thus prevent other misunderstandings.
I'm a retired police officer who investigated this sort of complaint and never let anyone know, even thru innuendo, who had called. To do so is unprofessional. You may have misinterpreted what the investigator said. You would be correct in assuming the information came as a result of what your son said at school and this may be what the investigator was agreeing with.
I've had personal experience of this sort too. A CPS investigator and police officer went to my granddaughter's day care during Christmas break to ask about her mother knocking her against the wall leaving bruises on her back. I was a playground volunteer and heard my granddaughter tell a couple of friends about this incident. The friends were very upset and came to me. I had witnessed the incident which wasn't as bad as it sounded (there were no bruises) and tried to reassure the girls. But, one of the mothers did call CPS. My daughter could've assumed that the day care called but I think it was a mother. It's been a year and there have been no further incidents. The day care is a good one and it is good that my granddaughter continues to go there.
Please accept that this was a false alarm based on your young son's story. You don't know who called and even if it was the teacher or someone related to the school, it was done out of concern for your son. It is always better to investigate in error than to ignore such a situation. I suggest that the person who called is inexperienced and probably called with good intentions tho that may not be the case. Do not be quick to blame the school. Anyone hearing the story could easily of jumped to conclusions.
I urge you to talk with the teacher and let her know how you feel while working to keep an open mind and with the goal of mending the relationship. Your son was doing well and has made friends. Isolating him from his peers is not helpful, especially after having this experience. Removing him may cause him to unconsciously feel that he mustn't talk about what is happening in his life. Resolve this issue with the school.