Thrombophlebitis happens when a blood clot causes swelling in a vein, the Mayo Clinic says.
This typically occurs in your legs, but can also happen in the veins in your arms or neck.
According to the Mayo Clinic, thrombophlebitis can occur in a vein near the surface of your skin or deep in a muscle. Blood clots in your deep veins can potentially cause more serious health problems, the Mayo Clinic states. These clots are at risk of breaking free and causing a pulmonary embolism.
Thrombophlebitis can be caused by periods of inactivity, such as being on bed rest or on long flights.