Either a ligament sprain, muscle strain, a bruised carpal (wrist) bone, or nerve impingement.
I won't advise you on anything medical, but if it was me, I would ice it, take some ibuprofen and maybe take a week off lifting. If it persisted, I would see my GP.
Care to answer a few questions? How severe is the pain on a scale of 1-10? How would you describe the pain in your own words? Does the pain radiate anywhere? Do you have any other sensations with the pain?
Also, how long's this been going on for/how long's it been a problem?