I consider myself to be pretty healthy, no diseases or disorders, 5'11" and 150ish LBS. I no longer do hard drugs, but I did for a while. I don't go hungry or thirsty and have a pretty balanced diet.... I get these little episodes, probably 2 or 3 times a month, and they seem to be random. I get them at work, at home, laying, walking, sitting. I get extreme tunnel vision, to the point where I can't really see anything at all, light headed and extreme chills, for like 10 or 20 seconds. And then it's over, vision goes normal, head feels fine, and I don't feel cold. This has been going on for about a year, and I didn't really think much of it at first cause I just thought it could of been side effects of the various drugs I was abusing pretty much every day...But I've been off everything except weed for 8 or 9 months and it still happens..
I'm no doctor but it sounds like it could be serious, have you been checked out by you doctor? if not defiantly get your self checked out before it gets worse.
I am also no doctor and hope i don't frighten you when I say this: go see a doctor asap.. what you are describing is a prominent side effect of brain tumors