I'm sure there are many different reasons why an ear can get red and hot...but the only one I've seen repeatedly is Lyme Disease...but please keep in mind that Lyme's about the only thing I've been reading about lately.
When your ear gets red and hot, is your earlobe spared? The most common cause of a red hot ear is a disease known as relapsing polychondritis. This is an autoimmune disease which causes inflammation of cartilage. If it involves the ear, often the earlobe is spared because it is not made of cartilage. Infections like Lyme disease causing isolated chondritis have been described, but they are almost always involved in areas where there are joints such as synovial joints (knees, hips, elbows) or non-synovial joints like the sternoclavicular joint, symphysis pubic etc. Lyme is a rare disease to begin with, and for it to cause external otic chondritis is even more rare.
The only rare thing about Lyme is doctors who have experience, knowledge, and expertise in diagnosing and treating it...and although it has been documented in every state in the union, some states have numbers that are now considered epidemic. In New York state alone, over 77,000cases have been reported to the State Department of Health since 1986.
One could discuss numbers/symptoms/diagnoses forever...the important thing here is to do the research for yourself. Never rely on any one source for your information...no one knows everything, and even if they think they do, mistakes can be made.