I experienced flu like symptoms when taking Sus-250. Only when I injected in my shoulder though. The doctor told me that it was my body reacting to a preservative in the suspension. I had a red circle around the injection site. The circle and the symptoms only lasted about 24 hours though.
As well as having a reaction to a preservative, sometimes people can be allergic to the type of vegetable oil used. It’s not common but it can happen. (They will not be allergic to the same oil taken orally, because it’s actually trace amounts of vegetable protein in the oil that trigger the allergy, and these are broken down in the digestive tract but not when injected. At least, this is the explanation given me by a doctor who is quite knowledgeable in immunology and in AAS use.)