Your story brought beck memories of the two times it happened to me.
The first time about 27 years ago, I had an Arab that needed a penn shot 4 times a day, 2 in AM, 2 in PM. When I gave him a shot, I know I assperated the needle, (pulled back the plunger looking to see if blood came back) It was fine no blood. I think when I pushed the plunger down to administer the medicine, I must have nicked a vein, and some medicine went in the vein. He flipped over backward then ran backward around the stall. I ran and called the vet, he was here in 7 mins, I was in the car crying, and by the time the vet went to the horse he was fine.
I did not give a penicillin shot for 20 years.
Second time about 3 years ago. When I started giving shots again, I was so paranoid, I would assparate the needle about every 5 cc's, I think all the moving around of the needle caused it to nick a vein again. This time it was the Clyde, he just ran back about 20 steps around his stall and was okay after that.
I don't mean to make this any harder for you, I told my story that maybe some one will read it and beware that giving a shot is best left to a vet or someone very knowledgeable.
I had given hundreds of shots, and I knew what I was doing. I have never given a penicillin shot since.
I am so sorry, I know how much you loved that horse.
The best time to plant a tree was 20 years ago, the second best time is today.