What Causes External Itching Of Vagina And Anus?

I have been fighting a BV/Yeast infection combo for about 4 months. About 2 months ago, I also began to get anal itching. I was checked for hemorrhoids, but there were none. Treatments for the infections have been somewhat effective, but it always lingers or comes back. I have also been checked for STIs and all have come back negative. I went into the OBGYN yesterday and had a wet prep done again. It came back negative for BV/Yeast. The Dr. told me that it is possible that since I only feel irritated externally, that the test may not have picked it up (as it is an internal test). My question is, it is possible for me to have an infection still, just externally, and the test may not have picked up on it? The anal itching is still undetermined by the doctor, but I am using Nystatin and Triamcinolone Acetonide Cream 2X for 10 days on both vagina and anus. Thank you.