I'm really embarrassed to ask this question, and I don't know if I'm just being silly or not, but since the two areas are pretty close, is it safe? (Female, 18 years old)

Probably safe. Are you worried about infecting the area or traumatizing it?
I think where you might have problems is if you have to tense up a lot while masturbating. I suppose that could tense up the anus too and you might get a tear.
Try to stay relaxed.

There is a chance that you will experience spasms, which are basically what an "O" consists of. If that is not an issue for you, then the act probably won't cause any problems. I know I am messed up for the next 24-36 hours after a release.