are you thin for your frame and height?
do you exercise and drink a lot of water? it could be a blood circulation issue, and if so, these things help. also, as someone mentioned, it could be an iron deficiency, but i wouldn't even worry about that unless you're given reason to. see what your doctor has to say, and stay active.
i find when i'm eating healthier (fewer refined foods, etc.) and my metabolism is quicker as a result, my circulation is also better and i don't get as cold.
i'm vegan too, btw.