Bald spot on my upper head, appeared rather quickly and on a random spot. What is it? (Photo)

Random bald spots can be Alopecia Areata (AA) if this was a multiple choice question.  You still need an exam to make sure this is not something else.  Scalp MicroPigmenation can help but doing NOTHING will likely help as well as hairs will grow back (if it is AA)

The cause of your hair loss is likely alopecia areata but you need to see a practitioner as there are other potentail more serious causes including infections which can lead to permanent hair loss if left untreated. If it is truly localized AA, I tell my patients 75% grow back in 12 months without treatments but this can be easily treated with intralesional cortisone.

It's a good idea to see your physician or dermatologist. There are 4-5 common causes of such spots, including alopecia areata, infections, trichotillomania, scarring alopecias.However, from a statistical perspective, the most likely diagnosis (by far) of a bald spot on the upper head that appears rather quickly and without many symptoms is alopecia areata. Others causes of round bald spots are more likely to have itching or different progression. Nevertheless there are a few things this could be and that's why a diagnosis is not possible without seeing the area of hair loss 'up close'If it is alopecia areata, don't focus much on the stress aspect. While stress might be contributory for a small proportion of those with alopecia areata, for most it has little role. Please see a dermatologist for confirmation of diagnosis and initiation of treatment. 

As this appeared rather quickly, I would make an appointment with a dermatologist as there are many causes of this that include autoimmune diseases like alopecia areata and fungal infections amongst others. You can not diagnose this over the internet.