From the picture and the symptoms you mentioned, especially the itchiness, it seems that you are right about it being that being eczema. Also, eczema is common behind the knees. From that picture though, it does not seem like that eczema is very bad. There are a lot of worse cases. Though there is still some dispute in the medical world, one study1 says that the cause of eczema is a bacteria that lives on the skin. This bacteria blocks the sweat ducts, which causes a reaction in the immune system that causes irritation, itchiness, and rashes.
WebMD2 says that it is likely that the immune system causes the reaction, but they also provide some other possible causes, such as genetics (eczema is usually common in families), certains environments you may be, and skin irritants like chlorine, makeup, and even some soaps and cleansers. It is likely that these skin irritants cause the bacteria mentioned above to react, which in turn cause the immune system to create rashes and irritation on certain parts of the skin.
Tips for treating eczema
Other than drugs that can be prescribed by your doctor, there are a few things you can do to help treat eczema.
Use more mild soap - some soaps can cause dry skin; these are the types of soap that tend to irritate eczema; try to get soaps that won't make your skin dry (usually your doctor can recommend a brand)
Showering - taking long, hot showers can also cause your skin to dry up; instead take shorter, warm (not hot) showers
Reduce stress - stress can be a factor for an irritation in your body; just relax sometimes; also, try to get more sleep
Petroleum jelly - using petroleum jelly on the spot of the eczema can help relieve the need to scratch
Moisturize your skin - dry skin is a side effect of eczema, so moisturizing your skin is important
 National Eczema Association - Study Reveals the Cause of Eczema
 WebMD - Eczema Health Center