What causes inflamation in the leg's vein. Does it cause pain...? What are the tests needed to examine the disease?

Varicose veins? Vein wall inflammation (phlebitis) is generally always painful especially if it is associated with a clot. Venous insufficiency (varicose veins), some diseases, certain medicines, an abnormal clotting tendency, and injury can cause phlebitis. Venous ultrasound will decide if you have venous insufficiency or not and can tell you whether you have phlebitis and/or a clot as well.
Phlebitis. Yes it can be quite painful even if clots are not being formed. Poor venous circulation, inactivity, having high levels of clotting factors, which may occur naturally, or seondary to drugs taken. A.Venous doppler will decide if you have venous incompetence and clot. It cannot diagnose phlebitis per se, unless used wilth a blood tests called d-dimer.