Some prostate cancers, particularly those that grow quickly, may not produce much PSA. In this case, you might have what's known as a "false-negative" — a test result that incorrectly indicates you don't have prostate cancer when you do.
Because of the complexity of these relating factors, it's important that a physician experienced in interpreting PSA levels evaluates your situation. You want to avoid unnecessary follow-up testing and treatment, but you want to be aware of false-negatives, so don't ignore warning signs.