It could be drug or food allergy. Ezcema could also be a reason.
Dengvaxia is available in some countries for vaccination and had some safety issues post marketing. ICMR has developed and has done trials on a tetravalent dengue vaccine which is supposed to release soon.
Applying menthol oil, Tea tree oil on the rash relieves itching.Aloe vera gel on the rash soothes itching(Make sure your skin isn't allergic to aloe). Try it on a smaller area before you apply on the rash. Ice pack, any deep mosturizer may also be used
Whenever there is a mosquito bite and spread of the Dengue virus, there is a chance of Dengue infection. There is no life-long immunity post first infection