A calf strain is a tear of one of the calf muscles which usually happens after a forceful movement. Symptoms include a sudden sharp pain in the back of the leg, often in the middle where the muscle joins the tendon. Swelling and bruising may occur.
A DVT is a blood clot in a vein. These are most common in the legs, especially after long haul flights. Symptoms include constant calf pain, tenderness at a deep point in the muscle and swelling may be present. It is important not to mis-diagnose this condition as a calf strain injury.
Cramp is an involuntary muscle spasm which causes a severe calf pain. The exact cause is not known but low salt levels, dehydration, low carbohydrate levels and tight muscles are thought to contribute.