Any chance you have Mulberry Trees in your pasture.
Had my horses trim the Mulberry trees on a regular basis for years, but during a
certain time of the year, they would get really swollen like you describe. Of
course, I spent a bunch of money on having a scope ran down to look inside and
various other vet bills for the vet to say "I dunno". Yes, have your vet
check the horse out, as there are many nasty things it can be, but also look for
what they can eat in the field, and consider an allergic reaction.
