The sensation of tickling or discomfort that you may feel when trying to hold your breath for an extended period of time is due to the buildup of carbon dioxide (CO2) in your bloodstream.
When you inhale, oxygen enters your lungs and is transported to your body's cells through the bloodstream. As the cells use oxygen to produce energy, they also produce CO2 as a waste product. The CO2 then travels back to the lungs, where it is exhaled out of the body.
When you hold your breath, the body's cells continue to use oxygen and produce CO2. However, since you are not exhaling, the CO2 builds up in your bloodstream, causing the blood to become more acidic. This change in pH triggers the body's respiratory center, which sends a signal to the lungs to breathe in air and release the CO2.
The sensation of tickling or discomfort that you feel is your body's way of telling you to breathe in air and release the CO2. If you continue to hold your breath, the discomfort may increase and eventually become painful, and you may also start to feel dizzy or lightheaded due to the lack of oxygen.