I'm no expert but are you resting a hand around your mouth when slouching on a desk when bored or covering your mouth because it could be due to the different dirt/oils placed around that area.It could also be due to a hormonal imbalance if it is a sudden outbreak.
It may very well be that you are sensitive to glycerin, which is found in many toothpastes. To avoid the pimples which result from your skin's contact with glycerin, brush your teeth BEFORE you wash your face. That way, any glycerin will be washed away before it has a chance to do any damage. Glycerin is notorious for causing pimples. Also, make sure to keep your phone away from your mouth when you use it. Phones are germ factories, in fact some studies have shown that public restrooms have fewer germs on their surfaces than are found on some phones!
You can get pimples anywhere on your body that has pores. What is happening is your pores are getting clogged with dirt or the like, causing your pimples. Wash your face at least once a day, with a warm washcloth :)