Purple fingernails and blotchy skin?

Im an 18 year old male who is pretty underweight. I'm about 6 feet tall and about 135 lbs. Whenever I get cold or nervous, my fingernails look purple-ish and my skin gets kind of blotchy. If I'm up moving around, they are red again, or if I'm not cold, they are fine. Or if i kind of move my hands around and rub my fingernails, they turn redish again. No injuries or anything, it's just been like that as long as i can remember.

Is this normal?

this sounds like Raynauds syndrome. It is when a person gets cold the capillaries go into a vasospam and limits the blood supply usually to the extremities. Very common to occur in the fingers. I had a girlfriend who had it and in the winter the same thing happened to her.

Sounds like poor circulation, or possibly anemia. You can get checked at the doctor's for anemia, or you can start taking b12 and iron supplements, but you should follow the directions. For poor circulation, you need to exercise. I have it too, and if I wasn't so lazy I'd be out there exercising too.

In old people, it is a sign of poor circulation. You need to eat more and better. You sound more like a skeleton than stylishly thin. It is normal for cold to affect the nails and skin.