Yes, unprotected anal intercourse with an infected partner can lead to infections or diseases, even if its one time!
Yes it can, some people think butt sex is a way to escape sexually transmitted diseases, but in reality, it is the best place to get std's.
If you are worried she had any std's, you should resume in getting tested currently, but also in another 4-6 months, as std's can take a long time to show up.
If you eventually find a partner who you know is free of std's and you can trust, anal sex doesn't have to be a negative thing, you just have to use lots of lube, and be gently at first, while it stretches. Some women simply don't enjoy the feel of it, but some women, once the initial stretching is gently worked through, absolutely love it. Sometimes, if they really don't like the pain, it might help to get the initial stretching done fast, with a shove.