A bout of urticaria (hives) can last for weeks. I broke out in hives over most of my body from some antibiotics in the middle of July and I still have them. Although they aren't as itchy now and seem to be slowly fading.

It may have been a preservative in the yoghurt, or a flavouring. If you still have the yoghurt, or know what brand it was, try to find and ingredient list and see exactly what was in it. Then check out this website alladditives and see if it is a known allergen. Trouble with allergies is it may not be the first time they eat something or even the 10th time, but it might be the 11th time they react kwim? Sometimes they slowly build up a resistance till they have an all out reaction.