Burning pain under buttock

[mistymoon729]

This all started in May 2015 I had a protected sexual encounter with a partner that I did not know their std status. Two weeks after the encounter I started have a pain that felt like a sunburn under my left buttock where my buttock meets my leg. Sometimes the pain is in the same area on the right. Occasionally I get a zapping feeling like a bug bite or a hair getting pulled. I have no rash. The first time this happened I was examined by three gynecologists, std tested at two weeks and herpes antibody tested at 1 month and 3 months. All negative. The burning pain lasted about two months and after the second negative herpes test I stopped worrying about it. Well now a year a half later January of this year it started again. Same burning pain. I got really paranoid again because it was reoccurring. I went to my gynecologist again who tested me for stds, yeast, BV, and herpes again, all negative. Ive also had an EMG on the nerves and an X-ray of my left hip. Im still having the pain now and I'm wondering if anyone knows what this is. Could my herpes tests be wrong? If not, why would this have started two weeks after sleeping with someone? Im miserable and I'm starting a new relationship and want to make sure I keep him safe. Please if anyone can tell me anything I'd really appreciate it. Thanks.