Hi there,
You've gone through all the correct steps! If your doctors have done all STD testing and ran other important tests, then you should probably take their diagnosis. If you still feel unsure, I would recommend seeing another doctor, although I feel that checking with three is quite a lot already for another opinion. How long have you been taking the Zantac 150? If it hasn't been very long, then I would wait for awhile longer to see if your systems gradually go away or get better.