Evening primrose is an Omega 6 fatty acid. It is important to maintain a balance of Omega 3 and Omega 6 fatty acids. I do not know where you are from, but in Western cultures (such as the US) diets are very, very high in Omega 6 f.a.'s throwing the balance off.
I would be interested in knowing more about why you are taking the primrose oil. I also believe it is better to get your omega 3's from food sources versus supplements as it is easier on your body to process.
Let me know what dosages you are taking and what your diet is like.