hello and welcome,
i suppose you are on a crash dieting since 15 days.
so your stomach and the intestinal flora has become accustomed to the low calorie diet.
suddenly when you give them a heavy diet which is very fatty, the intestinal flora will act as dramatically as the change in food. this could be one cause for your diarrhoea
but since the time between the intake of the food item and the symptom of diarrhoea is too small ( 15 min) , this could be a case of food poisoning
with pre formed toxins by micro organisms.
visit a general physician for your treatment. in the mean while keep yourself hydrated with plenty of drinking water.