The spots could be a sign of her getting sensitised to something. Or they could just be spots! Do they tend to come and go quickly? A normal spot will hang around for a while won't it?
I think it's a good idea to get it logged with the gp. Do you have any family history of atopic illness? (asthma, eczema, hay fever, food allergies) If so it raises her chances of developing it.
You could also start a diary, because it's very easy to forget the details, but they are necessary when trying to establish a cause if it happens again. Eg write down the food (all ingredients if you can) and reaction, and if the spots come and go, try to write down when and if there is anything in particular she's had in the 24 hours beforehand.
You could try to get a referral for skin prick tests. It very much depends on your area and your gp as to whether you'll be successful. They are by no means 100% reliable though (false positives and false negatives are common - dd1 tested negative to cheese even though it was the very thing that had triggered the referral! She did test positive to general cows milk protein though), so might not give you an answer. You've got an idea of what it might be - wheat, dairy, egg, tomato, sulphite (ham), pork, grapes, whatever ingredients were in the yoghurt. If you can dig out a full list of ingredients for the food that might be helpful - eg you might find that traces of nuts are in there too, etc.