Unprotected sex, even briefly, does expose you to various things. A one time exposure is low, however. If your penis was inserted (not just rubbed), you have a minimal risk of HIV which would require testing at 28 days or after post event with a 4th generation duo test. Then all the other std's are also a possibility, unfortunately. The contact of your penis to their skin puts you at risk. However, again, this sounds very brief which certainly will minimize that. You can monitor for symptoms and test or do routine testing. Did you know the person? If you know her pretty well, I doubt she'd secretly have HIV. And she'd likely not risk giving you an std if she knows she has one. she's 'less risky' if you will, because she is a known person. That's not to say it couldn't happen though.