Why iam getting irritation in sunlight

some people suffers from a condition called solar urticaria and its one type of allergy that happened when you expose to the sun light 
the exact cause of this allergy is not known but it results in activation of mast cells resulting in signs and symptoms of urticaria ,,and it varies from person to another 
how its diagnosed
usually by a dermatologist who will examine and take the full history from you and will do some tests like CBC and electrolytes , in addition to the allergy tests like exposing skin to different wavelength of light and documents any allergic signs ,, sometimes special skin patch applied and tested for reaction ,, skin biopsies would help in certain situations as well
signs and symptoms 
itching 
redness
burning sensations
hives
bolus
headache
nausea
low blood pressure
fainting
treatment 
Treatment for solar urticaria depends on the severity of your symptoms. 
Staying out of the sun may resolve symptoms if your reaction is mild.
sometime your doctor would  prescribe oral antihistamines or  over-the-counter creams, such as aloe vera or calamine lotion.which target histamine release and relieves the symptoms
some times corticosteroid and other medication like montelukast  and omalizumab  will help 
in sever situation you may admitted to hospital for monitoring and IV medications