Hi
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Heat radiating in the calf especially after a knee surgery could be due to clots in the deep veins of the calf or deep vein thrombosis. A duplex ultrasound or angiogram is required for diagnosis. Please consult a vascular specialist. This may need further treatment with medication and maybe even surgery. Please get this evaluated fast. Take care!
I would go see you PCP to check that out ASAP.
Could be a DVT do not mess around with seeing a doctor about it. If it is a clot, you need help right away.