Or maybe it's the bowl of cereal you eat in the morning.
A recent medical study published in The Journal of the American Academy of Dermatology indicates drinking skim and low fat milk (and certain other dairy products) is associated with developing acne. Scientists aren't sure why, but the link is clear. It's thought to be a result of an increase in certain proteins which cause an increase in insulin production.
Whole milk and full fat dairy products don't have the same effect. In fact, they actually help prevent breakouts as a result of increased bioavailability of vitamins A and D, known to be linked for healthy skin.