What Can Be The Reason For Hives?

I impacted my thumb with several hundred pounds of force recently. (4 days ago). The injury site swelled so badly that it looked as if someone had inserted a ping-pong ball into my finger. Swelling subsided within a few hours.

The day after, I started getting hives and it it still ongoing. My doctor has perscribed prednisone and hydroxyzine for the itching and hives but are unsure of the root cause. Is it possible that the injury is still causing hives? How long does this typically last?