I am very fit, train boxing three times a week. have just noticed swollen sore vein on inside of knee, is this bad? hurts to walk and pops out.

Knee pain: Looks like soft tissue injury just rest ice and ace bandage. If does not get better see your pcp.

That is: In the area of the superficial saphenous vien and if tender, prominent and at all reddish then there may be a clot in the vein. Not as important as if in a deep vein but should be evaluated by your doctor or a physician in a competent urgent care center or fast track of an emergency department. In the meantime warm compresses, and Ibuprofen and rest the limb.