40m since yesterday pain in back of thigh above knee for no reason. 2nd google result says dvt/blood clot. no redness, swelling, risk factors. strain?

Thigh pain: Your description is nonspecific. Possibilities include simple muscle strain, baker's cyst, contusion or phlebitis. There may be no redness or obvious swelling with any of these, but tenderness is usually present with phlebitis. If discomfort persists visit a hospital emergency room where examination by a physician and appropriate testing like an ultrasound is readily available.