I had very sudden onset fat loss or malar bags in face. What caused this and how do I get rid of this? (Photo)

Thank you for your question about wrinkles and bags under your eyes.  As we get older, we lose volume and collagen in the skin on our faces.  One solution that would address this concern would be injectable Fillers.  One product that is beneficial for this area, also known as tear trough, would be Voluma, Vollure, Lyft, or Defyne.  They are hyaluronic acid (HA) products that add volume to the cheek area.  By adding volume to this area, it will minimize the shadow that is giving the appearance of dark circles or bags under your eyes.   
To be sure fillers are right for you, see two or more experienced, licensed and board-certified providers in your area for a complete evaluation to make sure you are a good candidate and that it is safe for you to have treatment. I hope this helps.

Dear Toronto,It appears that you have evidence of fullness in the lower eyelid region and festoons/malar mounds on the cheek as well. These changes can make a person appear tired, sick and older even when they feel fine inside. 

Many people who undergo lower eyelid surgery to improve the appearance of their lower lids are dissatisfied afterwards because of one factor which can complicate the results from lower eyelid surgery.

Festoons and Malar mounds are a medical condition in which the skin of the lower eyelid becomes significantly damaged, usually by the sun. This damaged skin can result in folds in the lower eyelids and can lead to swollen mounds on the cheeks. They often occur with age and are more common in people with lighter skin types.

Festoons occur in many people seeking lower eyelid rejuvenation and if not corrected at the time that lower lid surgery is performed, the condition can become exacerbated and make the lower eyelids look worse after surgery.

The reason then why many people are dissatisfied after undergoing lower eyelid surgery to improve the appearance of their lower lids is the result of two main issues:

Swelling that occurs in the face after lower eyelid surgery often accumulates in the lower eyelid region and can take a while to resolve.
The subconscious brain constantly looks for balance in a face, and when lower eyelid fullness and festoons exist they are seen to belong together, as both are usually age-related phenomena. When only one is then corrected (usually the lower eyelid bags), the other is seen in even greater contrast.  The subconscious brain then focuses in on this condition and tells the conscious brain the face is out of balance.
Festoons and malar mounds have been a frustration for surgeons for many years. Thanks to the initial work of Dr. Sterling Baker (a person with whom I trained) and my own improvement and refinement on that work over the last decade, I now have a sophisticated, technically-challenging but effective treatment for festoons and malar mounds.

Many people don’t fully understand or recognize the differences between these two conditions. The first difference is in their cause or etiology; festoons are a result of sun-damaged skin on the lower eyelid and on the cheek and are believed to be affected by underlying contrasting muscle forces over the years of a person’s life.

In contrast, lower eyelid bags are due to age-related fat protruding through the skin in the lower eyelid region. New research has shown that fat can grow and atrophy during our lives in asymmetric ways. The growth of fat in the orbit around the eye along with atrophy of fat in the facial regions around the eye, as well as weakness in some of the orbital containing structures, contribute the fullness we see in the lower eyelid with age.

For some individuals, this fat shows up at a very young age and this is believed to a result of genetic changes in the development of the eye area in those individuals.

The second difference is the location on the face where they form. Lower eyelid festoons can be seen primarily on the cheek with some presence in the lower eyelid region.  Lower eyelid bags are located directly below the lower eyelid lashes and are noticed to become more prominent when someone with these bags looks upwards.  Conversely, festoons are minimally affected when a person looks upwards.

The third difference is the feel of these conditions when they are touched. Festoons are squishy and can be made to move side to side. Lower eyelid bags are firmer, deeper in location beneath the skin and can’t be moved easily side to side when touched.

 It is important to understand the differences between these two conditions. The best rejuvenation and most natural results occur when both eyelid bags and festoons are corrected simultaneously.

I'll include some links below.

I hope this was helpful. 

Fillers are a great tool to disguise under eye irregularities.  The treatment is performed in the office setting and the results last up to one year.  Malar mounds are harder to treat.  Sometimes patients find improvement with the use of lasers.  I recommend visiting with an experienced injector or board certified plastic surgeon in your area to determine what treatments are best for you.