I have swelling on my right leg from insect bite, feels hot to the touch and is itching. I'm a diabetic. The area is also red with burning feeling.

Antibiotics. I would get to the doctor as soon as you can for antibiotic treatment. The injection can progress and travel leading to more severe complications. A cold compress might help now until you can get to the doctor for evaluation and treatment.
Many possibilities. Some causes of swelling can include systemic problems such as hypertension, and problems with the kidneys and localized issues such as arthritis, infection, lymphatic obstruction, blood clots, varicose veins, trauma (fracture, sprain, tendonitis) and side effects to medication. Have it evaluated by a professional and get the appropriate treatment.