What could be wrong with my little sister (seizures... drugs possibly)?

My little sister is 17. I haven't seen her in a while ( I just came back home from school) and she's lost a lot of weight (25 lbs or so since Christmas break). I was eating lunch with her the other day, and noticed her hands were shaking as she held her fork, but I didn't say anything. Then today, we were driving to the supermarket and her whole body was trembling in the passenger seat and her eyes got really big and spacey. Also, I've been home for only a few days and I haven't seen her sleep yet.

I've always looked after my sister (Mom has some drug issues herself), so I'm not sure if maybe my sister possibly could be starting. Or, if it might be some sort of seizure. Not sure what it could be or what to do, I'm not her dad or a doctor, just her 22 year old big brother. This kid is my life, so any suggestions and support would be awesome.