AWE poor thing, I hate vax time. My horse as he got older started to have reactions, so I give him bute the day before, and the day of, and that seems to do the job.
My question is why do you think it was the rabies? Is that what the vet said? My horse reacts to rabies, strangles, and I think west nile. Maybe they used a different brand of vax this time around? Two of the horses at my barns necks got huge after their vax and they had to pick their food off the ground for them to eat.
You could also split the vax up, my vet came out for my horse gave him strangles, and than left me the other ones to give to him the next week.