Why do i only get oral cold sores after drinking alcohol?

Because it is: Stressing your body.

Immune suppression: Cold sores are often from a viral infection, and alcohol suppresses our immune response to viruses. You should see your doctor to determine if your sores are viral, and if so, have them treated more directly with the right medication. If alcohol causes you problems, but you still drink, you also need to consider whether it has become more of a problem in your life than you might think. Talk to doc.