She is 15.
She is not ALWAYS how I am describing, but it is enough to worry me... so....
So. l do not live with her anymore. Two weekends ago, she came over for the weekend. She saw my new cell phone and freaked out over how awesome it is. It is a pretty distinctive phone, white, touchscreen, full keyboard. But then the next weekend (few days ago) she came over again and acted like she had never seen it! She was tweaking over how cool it was. l told her she saw it last weekend, but she told me I am crazy. l do not get how she would not remember that. Anyway she is done a few things like that.

But she will remember stupid things like half waking up once in the middle of the night to me telling her to move over.

Anyway. Should l be worried, or is she just ... spacy?

This is normal, she is just flighty. There is probably so much going on in her life that she does not remember everything, also she may not remember everything that you do and vice versa. Do not worry about it unless it effects her daily life and she is forgetting things like her address and name or the date and who the president is, if she begins to forget these then call your physician. Good luck

Anything further.

that is weird. get her in have a mri on her brain. see a doctor. how are her grades in school. ?

There r several reason(s) that come to mind.
Is she using drugs? that is one thing.
Second, is she one any kind of medication or is she stressed out due to something tragic that has happened in her or someone she is close to?

l would sit down with her & have a heart to heart & see for yourself, that might be the very thing that she is wanting anyway. Some time to have with u & she does not know how to have a conversation with u now that u aren't' living in the same place?

take her to a doctor.

Every body for gets once in a while but if u sister continues to for get then may be consider seeing a doctor it could be serious under lying problem. What l mean by continuing to for get is like if she puts some thing down & walks away from it. Not remembering about it & it is all the time.An example, is like u give her a box of Ice Cream to put in the freezer. You can not do it because some thing came up real fast. She says she will put it in the freezer but realizes her shoe lace is untied. She bends down to tie her shoe lace puts the Ice Cream on the floor beside her. Ties her shoe & leaves with Ice Cream on the floor. Not remembering at all she left it there but be care full not to mix up normal teen age flightiness years has a problem that needs fixing.. Time will tell who will need help !

You need to ask more questions about her life to learn if she has the same problem in school or at home.Ask her if she is taking drugs or anti depression medication or if she is eating right.

Record her secretly when she visits u & play it back if she forgets what she has done to convince her that u r not imagining things.

If she has frequent memory losses l think a visit to the doctor would be in order.

She could be using the amnesia excuse to visit u because she feels lonely at home or too stressed.