It's good that your doctor did the confirmatory test to make sure you didn't have syphilis. There's a long list of things that can cause a false positive on an RPR, like pregnancy, autoimmune disorders (especially lupus, but some others), IV drug use, malaria, tuberculosis (TB), lyme disease, certain types of pneumonia.
Some people never figure out why they did, and a 1:1 could really just be a fluke. I couldn't find anything about herpes causing a false positive RPR, even during an outbreak, but that doesn't mean it didn't cause it.
How long ago was your most recent possible exposure to syphilis? It can take some time to test positive. If you've had oral, vaginal or anal sex in the last 6 weeks, it might be too soon. A syphilis sore usually appears at about 3 weeks, though it can anywhere from 10-90 days.