Wow, Sarah, your story blows my mind. Truly, I don't know what to say to you about it. This is indeed a weird mystery. However, as another prolific responder here, verryworried, says, "nothing about this virus surprises me anymore." Having said that, this indeed surprises me. I have no idea how you'd have gotten HPV 18. My first thought is that this has to be an error. Is it possible to repeat this test? The reasons your doc gave show she's equally clueless and grasping at straws for an explanation. There is just no readily available way to say how you'd have tested positive. If possible, repeat the test.
Having said that, HPV really does usually clear up on it's own and rarely causes any issue for women in the long run. Even HPV that leads to highly abnormal cells on the cervix and then after than progresses to cancer rarely causes any issues because it takes years and years for this to occur so it's caught at dealt with long before that. So, in the grand scheme, you've nothing to worry about. You'll simply continue to go forward with your check-ups and see what's up.
Wish I could help more, but your situation is truly unique. Other than a re-test, I don't know what to say. Best of luck and don't get to weird about germs!
This is very unusual. I highly suggest getting a second opinion. Her suggestions as to how you might have gotten it are bogus. Genital to genital, mouth to genital, or "maybe" hand to genital are the only ways. Please get a second opinion.