How often is "a lot"? Like every day/several times a day, or just after feeding, or every couple of days?
Are there any signs of mucus around the mouth or nasal opening? Any wheezing, open-mouthed breathing, or is the head constantly held up above the ground?

After feeding sometimes snakes yawn to get their jaws back into place. That would be my first guess, but I am not an "expert". I do know that it would be helpful for other more experienced people to know the answers to the questions above - and maybe a few more?