Why would she fake this to get attention? If she wanted attention, there are a lot more obvious things she could fake, such as pain, vomiting, etc. Don't accuse her of faking without proof. It may make her less likely to come to you when there is something wrong.
You should take her to the doctor if it doesn't stop. It could be a nerve twitch, which are very common and would probably go away on it's own, or it could indicate something like Turrets syndrome, although that's probably very unlikely based on just a nose twitch. It could also be from stress.
If she is nervous about her period coming in the middle of the night, have her wear a light pad when she goes to bed. Tell her that if she does bleed on the sheets or her pajamas, that it is not a big deal and it can be cleaned up. Once she gets used to having her period, she should relax a bit more about it. It can be stressful early-on.
If she is a nervous person in general, it may be good to try to teach her some relaxation techniques while she's still young so she can manage her stress on her own in the future. Focusing on her breathing and taking deep breaths, exercise, meditation, etc. When she's stressed, you can also have her engage in peaceful/calming activities like drawing, playing with a pet (if you have one), listening to quiet music, reading, or something else she enjoys. Also, try not to put too much pressure on her and lower your own stress level--if you are stressed about bills, or work, or whatever, she's going to pick up on that.
I hope her twitch goes away soon. Good luck.