Deep vein thrombosis. Commonly, blood clots cause pain and swelling but sometimes they will only cause one or the other. Occasionally, they cause no symptoms. If a blood clot is small and in what we call calf veins, you might only get pain without swelling. If the clot is only blocking part of the vein, but not all of it, you also might get no swelling.
Pain, swelling. A deep vein thrombosis in the leg causes swelling because blood doesn't return to the heart normally and there is pain from the chemicals elaborated from a blood clot. There is tenderness in the calf and pain when you dorsiflex the foot (lift the toes). It is dangerous because there is a risk of the clot breaking loose and clogging the pulmonary artery to the lungs (pulmonary embolism).