After my haircut today I discovered I've got some bold spots on the back of my head. How come?

Alopecia areata. A smooth bald spot is probably a condition called alopecia aerate which is am allergy to the hair which stops it from growing just under meath the surface. Fungal scalp infections involve the skin with scaling amd swelling. Treatment for alopecia aerate is localized injections of medicine once a month by dermatologist.
Bald spots. Could be alopecia areata, an autoimmune condition.
Bald spots. Not knowing the details it is difficult to assess your cause. Most common reason is physiologic male pattern baldness, however patchy baldness could be secondary to fungus or secondary to autoimmune causes. At other times no specific cause can be figured out, but these recover spontaneously.
Possible infection. Hair loss in a patch or spot can be from a fungal infection of the hair follicles. Best to see your family doctor, internist, or dermatologist to confirm the diagnosis. Fortunately, this type of infection is easily treated and hair should re-grow once the infection is treated.
See your doctor. It could be alopecia areata or a fungal infection. See your doctor to determine the cause. It is not an emergency but schedule an appointment to see which of the two it is. Alopecia areata is related to stress.