As diagnosis of urticaria is made, remember that most of the time it is not serious.
But search for the cause. ( insect bites, dust, pollen, viral infections, food, medicines and many other causes ) in order to avoid recurrence.
The rash is usually transient, but may recur upto a week or two.
Next Steps
As child is 5months, atarax and other anti allergy medications are best avoided .
Health Tips
In any allergy, keep in mind the red flag symptoms ( rapidly spreading rash, swelling of eyelids, lips or especially tongue, coughing which are all serious and need urgent attention at the emergency room )