Any food if stored and becomes old might have growth of fungus on it and consumption of such food will give rise to allergic reactions leading to hives and rashes. But before blaming the rice would you please let me know what other foods you had and where were you during the past 6 hours before you got the rashes.
with best regards,
Now that its clear on what you took i will dwelve deeply.
Looking to the foods taken there are many which might give rise to a reaction in your body. First i would suspect the Mushroom. Next in line would be the smoked salmon as there are chances of other allergents mixing with it while being prepared. i suggest that you take a course of anti-histamines for three days and refrain from taking any oily / smoked food / fish and keep your diet mainly on salads [cucumber], juice of bittergourd [added with honey], and drink more warm water, tender coconut to flush your body. avoid salt intake too. Drinking light live yoghurt will help preferably added with garlic. take care.