According to the Centers for Disease Control and Prevention (CDC), 1 in 5 Americans has a sexually transmitted infection (STI), which can lead to sexually transmitted diseases (STD). Given that high frequency, it's not improbable that a sexual partner could contract an STD. And while a positive STD test could raise suspicion—perhaps indicating infidelity by your partner—it is important to understand your partner may well have been asymptomatic the entire time you've been together and never knew to get it treated.
Regardless, it's important and necessary to take steps to ensure the health of both you and your sexual partner(s) once the situation becomes apparent. The most important step is to seek medical advice if you suspect exposure and/or symptoms and to let all potential partners know so they, too, can seek medical care.