You describe a sexual event with little or no chance of STI transmission. Oral sex is not risk free but quite low risk for all infections. (Cunnilingus -- oral-vaginal contact -- comes close to zero risk.) And although you say nothing about your co-worker's sexual lifestyle, most likely she is at low risk. (Even if she has her own non-marital sexual experiences, if those are like this one, she is unlikely to have any important STI. As for your wife's symptoms, vaginal area itching is extremely common in entirely healthy women and is unlikely due to any STI. And she is exactly right about such symptoms becoming increasingly common as women age -- if the problem continues she might discuss it with her gyn, perhaps expecting to learn she's getting low on estrogen as she approaches menopause. Finally, your absence of symptoms also is reassuring -- despite "some discomfort" that you attribute to stress (probably correctly!).

From a medical/risk perspective, I see no need for either you or your wife to be tested for anything.