Large painful vein

I've had this big dark blue vein behind my knee where your leg bends forever but I kinda forgot about it till it started hurting again when i'm kneeling down working. I tried googling similar things and it seems to be varicose veins but seems unlikely because I don't really do anything to cause it. I work out regularly so my circulation should be good but I also smoke cigarettes and marijuana. I kinda stress about things related to the cardiovascular system. .. Was wondering what it is and if it has any threat to me what so ever
Source: MD Health Forum - Medical Discussion Boards - Category: Consumer Health Advice Authors: Tags: Internal Medicine Source Type: forums