Brief Answer:
possible deep vein thrombosis

Detailed Answer:
Hi,

If there was no injury or insect bite then this may indicate deep vein thrombosis and this required further diagnostic and management. It happens when vein become occluded by thrombus. I suggest to see doctor, do doppler ultrasound and check d-dimers in blood. Till that, apply elastic bandage, avoid long standing and keep leg elevated when laying. If deep vein thrombosis is present then you may need heparine therapy.
WIsh you good health. REgards

---

Brief Answer:
rarely

Detailed Answer:
Hi.
Such risk is minor ad you should not be worried,but attention is required because in severe cases it may lead to lung embolia and breathing difficulties.
It may occur after long sitting, for example during long flights, after leg inuries or as a result of some blood clotting disorders.
In most cases it pulls back spontaneously, but as I said, it should be confirmed first by doppler ultrasound.

---