What you have sounds most like something called a corn. A corn is a collection of dead skin cells that form a round hard callus. They occur when there is consistent pressure or friction over a thin of skin. They occur most often on the feet, but can occur on the hands and often occur on the thumb when they are on the hands. They usually only bother people when they come in contact with something else. They can be painful in some instances. The treatment for this would be removal with a small knife (by a doctor
Another possibility is that you have developed a wart on your thumb. Warts
are small growths that occur as the result of an infection with a virus. The most common type of virus to cause a wart is the human papilloma virus. These are treated with cryogenic freezing. This is where a doctor takes liquid nitrogen and freezes the wart causing it to die and fall off. Warts are not typically painful.
Since it is not clear what you have, you should start off by scheduling an appointment with your primary care physician
. He or she can examine the bump on your thumb, determine what it is, and then determine the best mode of treatment. Good luck.