Professor dismissed error I found in her work. I need more feedback, but the field is small Background / My problem:

I believe to have found a substantial error in my professor's original research.  After getting answers to my previous question, I emailed her about it, but she was dismissive. Of course, I could be wrong and it is very possible that I am. My professor seemed not interested at all to talk to me though, so I cannot discuss this with her further.

It is not a factual error, but an error that concerns the logic of her argument. To simplify, I think that she says that both P &amp; -P. This is not obvious at first, and it becomes apparent only after some entangling of her whole argument.

The field is small:

Now, I do not personally know anyone who is interested this field. The field is very small and I can assume that pretty much everyone knows my professor and that my professor knows pretty much everyone too and that they talk.

What do I do now? / How do I get feedback?

Ideally I would like to get feedback from other experts. Maybe I am wrong. I am happy to be wrong, but I need to understand why. I could maybe email them, but I am not sure how responsive they would be. 

Also, I am worried now that if I ask other people to have a look, my professor will hear of that. She will then know that I did not take her dismissal seriously, and am working on a paper. How would I go about this? I am worried to look like a backstabber, esp. if I email other people and say "Look, I believe I have found an error in Professor XXXX work" after my professor already told me it is not an error. If I do something like this, I would also be especially worried that should I be wrong, all of this will be very embarrassing for me.

PS: This concerns original research, not a textbook. I am in the Humanities. Please give me a chance, even if you believe that I am probably wrong,
In this case, you should consider iterating with you professor again. Perhaps in person, rather through email (the explanation could be lengthy and that might be the reason why your professor was dismissive). You should also consider changing your approach, try to assume that the original research is correct and from that standpoint approach your professor for an explanation (again a situation when a personal approach might be better than an electronic one - the tone of your email might be misunderstood). You can then guide the discussion toward the issues you see.

If all the above fails, I would still hesitate to go to other people (whether in the field or not) with the attitude "My professor made a mistake and refuses to discuss it with me, I need you to confirm for me that I'm right". You course in this case is to learn as much as possible about the area and determine for yourself whether or not there was a mistake. During these actions you can and should ask and discuss your work with as many people as possible.

With original research it is often the case that even experienced researchers need to follow the above path (although their's is shorter than a student's) to state whether or not something is wrong.