My gross little fingers really want to pick at that If the vet is coming might as well wait though.
It could very well be the cause. My horse (who has never had an issue of any sort) had a very small area of scratches (or something) on the back of his pastern and we only noticed because his leg ballooned up halfway up his cannon. No heat no pain except for the very immediate irritation.
I wouldn't worry too much, wait and see what the vet says.