Oral sex exposes you to herpes, syphilis and gonorrhea.
-Herpes typical lesions are visible on the skin, so if you didn't see anything on that lady's skin, probably she didn't have it, or even if she had it, she would be unable to infect as your skin didn't contact the lesions.
-Syphilis, there are also typical lesions called "chancrus" always in the place where infection starts, but not always on the skin, they can also be inside the body (but as there was no penetration there was no risk).
-Gonorrhea, if you didn't see any white discharge from her vagina probably she didn't have it.
But sometimes, STDs don't show any symptom, so in this case, is good to test for them after three weeks. In the case you get a negative result, would be good to test again after three months, because there are people that take longer to show a positive result.
You could also think about chlamydia, but it is not known to be transmitted through oral sex.
In general I would say the risk is really low in general.
I waited over a month to get tested. It's horrible and depression having to wait. I get my results in tomorrow.