According to the Mayo Clinic, thrombophlebitis is when one or more of your veins swell because of a blood clot. These usually occur in your legs, the Mayo Clinic states.
Thrombophlebitis can be caused by extended periods of inactivity, such as a long hospital stay or sitting for too long.
These blood clots can develop in a vein near the surface of the skin, or they can also be in your deeper tissue, which causes deep vein thrombosis (DVT), according to the Mayo Clinic. DVT can lead to more serious health problems, including pulmonary embolism.