No one really knows what causes Rosacea, although there does seem to be some relationship to both bowel problems and hormonal imbalances. I tend to find that it flares when a number of situations all come together, if one is stressed, the diet is often poor, too much caffeine can be drunk and the combination of diet and stress will affect both the bowel and the hormones. Skin problems are always caused by a combination of factors.
Rosacea is made worse by exposure to sunlight, eating spicy foods, drinking coffee and often by red wine. All of these heat the body up and it irritates the skin. Blood cleansing herbs such as Sarsparilla and Burdock can all be used to treat Rosacea. Depending on your age, I would combine these with some hormone balancers such as Red clover. Napiers Skin Soother Herbal Relief combines these herbs in a traditional blend. I always treat the bowel when working on the skin, and Slippery Elm would be my herb of choice here. It acts as a demulcent, putting a protective coat on the gut and helping to correct the bacterial balance.
You also need to get your skin condition properly diagnosed as it can be confused with other factors such as eczema or allergies.