What can cause swelling, redness and pain in one leg?

Possible blood clot: My main worry with those symptoms would be a blood clot in the leg veins called a deep venous thrombosis (dvt). Causes can be long trips/immobility, recent surgery and inherited conditions.These can be life-threatening so need attention. Infections such as cellulitis can also cause those symptoms, as can gout (not always the toe), a baker's cyst behind the knee, and other things.Maybe need checked.