Answer:
Hypoventilation is causes an increase in CO2 (carbon dioxide) in your blood. This is caused by the lack of breathing, or obtaining oxygen, (on the contrary, hyperventilation is when your body is gaining too much oxygen - people tell the hyperventalator to breath in a brown paper bag to allow CO2 to bind to hemoglobin). Thus, when people hypoventilate (i.e. causing an increase in CO2) this causes the pH in your blood to decrease, making it more acidic. (and when hyperventilation occurs, O2 is increased, which causes an increase in pH - making blood more basic).