4 years ago- bad bone bruise and sprain of hand (Between thumb and first finger).Since then, if I use it a lot (writing)pain returns. What could it be?

Rehabilitate. If you didn't properly rehabilitate the bone and joint then it's possible that you are left with long-standing pain. However if you start exercising the area again then it's quite likely that the pain will permanently disappear.
CTD. Cumulative Trauma Disorder is a condition I see in my sports medicine practice in which an injury to a joint is compounded by occupational repetitive strain of overuse. You sustained an acute injury to the thumb and index finger of your dominant hand aggravated by repetitive strain of writing and keyboard work. You may have post traumatic arthritis or tendinitis. Try NSAID creams, rest and ice!