When exercising your respiratory system works harder and faster because oxygen is being consumed and carbon dioxide is being produced at an increased rate by the cells in your body. Your heart rate also increases to try to increase the blood flow. The blood is what carries the oxygen from the lungs to the tissues and the carbon dioxide from the tissues to the lungs for expulsion. So during exercise, the carbon dioxide levels in your tissues will increase as a result. Since there is an increase in CO2 in your system, this will also lower the pH in your tissues since CO2 will dissolve in water to produce small amounts of carbonic acid (H2CO3). And an increase in acidity lowers the pH level.