Welcome to Just Answer and thanks for your question. The only way to really tell if it is a blood clot or not would be to have your physician examine it. Some clots will be red and hot, but they are difficult to see if the whole area is red and swollen. Having it on one side and not the other is also something you see with a blood clot. If you have any doubt about what it is, you should see your doctor right away.
Either could cause swelling. It could also be phlebitis (inflammation of the wall of the vessels) or cellulitis (infection), venous insufficiency. It will require an examination to determine what it is.