Allergy is a definite possibility, I agree, in the absence of pain and if there is no redness also. The steroids are very unlikely to cause allergy but they can have allergic reactions. Taking anti-allergic medication like benadryl, if helps, will further substantiate this and will give a therapeutic diagnosis.
The other causes like infection or affection of the venous valves (thrombophlebitis or phlebothrombosis), lymphangitis, are associated with redness, pain and fever, apart from swelling.
Please feel free for your follow up questions.
The ultrasound is very good technique for detecting the stenosis of the artery and its blood flow. This also detects the deep vein thrombosis which is called as phlebothrombosis. Arterial stenosis will give lot of pain and color changes in the arm like blue or black. Did you inject it yourself? Phlebothrombosis too give quite serious symptoms; especially the pain, redness and swelling are profound.
Since the origin of the drug is somewhat not standardized, this is the reason, I suspected allergy. Low grade/mild grade infection obviously is a possibility. I read this resource before answering you;
Do you want me to read something else or specific site of manufacturer, which you used?
Please feel free for your further queries.
The only other possibility seems to be is a chemical reaction which can induce inflammatory reaction in the muscles and lymphatics. The medication or mode of injection cannot cause lymphatic edema as seem by breast surgery or surgical dissection of the lymph nodes in some diseases/surgery. The symptoms of infection can be masked by the antibiotics and medrol pack as they reduce the inflammation/infection tremendously (this also needs consideration).
Swelling can take 5-7 days, to go. So, taking same treatment for further 3-4 days would be prudent. You are right that antibiotics give significant improvement in three days. Medrol pack helps in infective and allergic, both the conditions. In the present context, it may be difficult to give you exact diagnosis (allergy or chemical inflammation are more likely), the best course seems to be continue with the same treatment regime.