My monogamous girlfriend's vagina usually smells pungent a day or two after having unprotected sex. could this mean she has another partner?

Not a reliable sign: If you are concerned, ask her! Openness in a sexual relationship is essential to a good relationship.

NO: Most likely not-but vaginal odor could be a sign of bacterial vaginosis. It is NOT a sexually transmitted disease. It is impossible to make a diagnosis of vaginal odor by history alone--need speculum exam, cultures, microscopy and likely all of the above