I Have A Question About STD's/STI's. My Girlfriend Has Had

I have a question about STD's/STI's. My girlfriend has had unprotected sex vaginally and anally in her past relationships. She and I have both been tested and are clean, and have had unprotected vaginal sex with no issues.
But my question is, if we have unprotected anal sex, could she possibly have an anal STD that she didnt know she had?