From the description you have given it seems more like urticaria
than contact dermatitis
. In most of the cases, urticaria has no underlying cause. However worm infestation, particular food intake, foods with preservative like jams pickle and sauce, medicines, infections can cause urticaria. To treat the same, you might require a course of oral anti allergic like fexofenadine
for couple of weeks. You can also apply calamine to soothe rashes. Hope that answers your query. Take care.