For some reason everytime i even brisk over my left under boob it's painful. i can't even press down on it without feeling a burning sensation. i'm thinking it may be gas, not sure!

Not quite right: Gas wouldn't cause a very superficial tenderness like you're describing. Rather, it would usually cause abdominal pain which is worse with pressure on the belly. It's common in women with large breasts especially who wear underwire bras to develop rashes or abrasions from pressure in that area. Check a mirror for signs of a rash. Consider well-fitting, comfortable bras. Keep the area dry.