Hi, when you had that episode, the injection would have missed the vein and went into subcutaneous tissues or the layer under the skin and that's why that part of the skin would have swollen up with whatever you were injecting. You can apply pressure bandage over it and take analgesic medicines for the pain. It will all go away in a few days. But it is advised to avoid such misadventures in the future as if by mistake you inject it into an artery it could be lethal. Take care,
Dr Rishi, India.