My first thought was also a sting or bite of some sort.
I don't know when it happened, but if it was a few hours ago or more, and she isn't reacting badly, then it should be pretty safe to eliminate an adder bite as the cause and make it more likely to be an insect bite, perhaps bee, wasp or horsefly.
Another thought, could it be a grass seed and the initial yelp was when a barbed seed of some sort entered her ear? I think that's less likely than a sting because she would probably be shaking her head a lot if there was a grass seed stuck in there, or carefully holding her head to one side.
Hope she's OK now.