Well, I don't think it is cancer related, but you could have bruised tissue, bruised muscles deep in the chest, broken ribs...only a doctor would be able to pinpoint the cause of your pain, but I would think getting kicked by a horse could cause any and/or all of those things.
Are you on any immunosuppressive therapy that would slow healing time? Bones can get infections in them, if they are damaged.
Only you can decide if a doctor's visit is warranted. Good luck, and hope you feel better soon.