This could be caused by a number of problems and it is difficult to know which one for sure without knowing more about your history and doing a physical exam. However, focally tender veins can be indicative of thrombophlebitis (actually inflammation of the veins themselves), deep vein thrombosis (DVT), or superficial vein thrombosis; all of which should be evaluated by a physician and treated if necessary. Hope you are feeling better.
Any number of conditions can cause this! I suggest seeing a Vascular doctor to get it checked.