What to do if i have severe pain in vein top right leg. what is it?

Have it checked: If a vein is painful, it may be inflamed - which can be associated with a clot within the vein - known as thrombophlebitis. If it is on the surface of the skin then it could be a superficial thrombophlebitis - less worrisome than a deep vein clot, but can be associated with deep vein clots. Sometimes veins are inflamed because they are varicose. A vein specialist will help figure out the issue.