If they're on your actual lip, it's probably cold sores which can be caused by just about any kind of common cold or a fever. I get cuts in the corners of my lips, caused by a lack of vitamin B in my diet or if I drink excessive amounts of citric acid; pop, lemonade..anything that's acidity really. If the sores aren't taken care of right away, they can be very painful, esp. if they start to heal then crack again.
If this is what you're talking about, decrease your acid intake and use a lip ointment for dry, cracked lips. I prefer to use aloe vera (from the actual plant) or take a vitamin E capsule and squeeze a very tiny amount on your finger and apply to each sore. You'll see improvement w/in a week w/the vitamin E.
Btw, for your stomach problems, you could drink soymilk instead. I am lactose intolerant and have switched to Silk brand, vanilla flavored soymilk. It actually tastes pretty good and I still can have my cereal, now whenever I want without worrying about cramps/gas. You might want to try that sometime :)