Methylprednisolone is a corticosteroid and not an AAS. It helps with inflammation.
It will help you with the symptoms from your sinus infection and will not interfere in such a way as to make it unsafe alongside your Anabolic Androgenic Steroids (AAS) use. Common sides are aggression...increased/decreased appetite...frequent urination...increased thirst....trouble sleeping.
Don't sweat it.
But be sure to take as directed. Prednisone has been shown to cause avascular necrosis (really bad shit) ins some people. Using in a method other than as directed may or may not increase this risk, but doctors always tell me that it will increase the risk. I have found no information on that, but I take their warning.