I just ate milk and cereal and my stomach feels like it blows up and i get really tight tight under breasts.

Lactose intolerance : Bloating after eatting cereal and milk is suggestive of lactose or gluten intolerance. Both are very common. To know which you could try just drinking a glass of milk or having your cereal with coconut or almond milk. Once you know what's causing the problem there is a lot of information out there about dealing with it. Best wishes.