What Could Cause Painful Swelling From My Thumb Nail After It Has Completely Healed From An Injury?

Mashed my (Thumb) nail in a dresser drawer back in Feb. It turned white, yellow, green, black. Nail finally grew out to the end taking 8 months and I cut the nail short. Now I have a swollen finger & much pain again after a week from cutting the nail. What is wrong? I ve used Hydrogen Peroxide but the pain is much. What do I need to do.