Try this little simulation : Press your index finger (normal pressure, not extreme pressure) against the floor or wall. Repeat it for 20 to 30 times. Or until you feel slight pain between your finger joints.
Chances are, you will start feel a little pain after pressing about 20-plus times. This is an example how repetition of the same activity over a short period of time could cause stress to your body. This is especially true for small and nimble parts like fingers and thumb.
The pain caused by such repetition of activity is called Repetitive Strain Injury or RSI.
In a day, we would probably used our thumb (and index finger) for thousands of actions a day – brushing the teeth, opening bottle caps, turning on a switch, clicking that mouse, tapping that touchscreen. Being the most used part of the hand, the thumb and index finger are more susceptible to RSI than other fingers.
If your index finger hurts when pressed, it could be a sign of RSI if you have no history of injury and have been recently overworking your fingers. By trying the first simulation exercise above, you will be surprised on how easy it is to hurt our fingers with RSI. Most will not even realize the injury until after it persists for a long time.
RSI, as its name suggest, is a type of injury and should not be ignored or taken lightly. Seek professional advice if you find yourself suffering from pain due to RSI.