It is VERY possible you don't have herpes. Plain and simple a visual diagnosis is not a valid one.
Simple solution though. Request an IgG type specific blood test.
From my understanding, it would be more common for you to have acquired HSV1 genitally (from receiving oral sex) and to not have another outbreak after that initial one than for you to have acquired HSV2 and have only one outbreak. If it truly was an herpes outbreak 20 yrs ago, antibodies, developed soon after infection, would remain in your blood.
So what you can do is speak with your primary care doctor or specialist about getting your blood drawn for a Herpes IgG type specific test that will give you numerical results for both HSV1 and HSV2. You do not need to have visible symptoms to get this test done.