Yes, any corticosteroid, is used to treat inflammation. And it is administered in many ways, even per Asthmatics, it is inhaled.
Being your son's croup was so bad, the Doc did the steroid. But in a shot, form.
Yes, it can make one more hyper.
It is temporary.
And yes he is over-tired from being ill and not being able to sleep. Over-tiredness, also causes a child to be more 'hyper' sometimes.
You might want to ask your Ped, why the shot? And not some inhaler version? Maybe because your son's Croup, was acute? And shots work, fast.
Again there are MANY different kinds of Corticosteroids.
They are not all the same.
You might ask your Ped, WHAT steroid exactly, was given to your son?
My son had Croup once. It is an inflammation and constriction of the Larynx and Trachea, in children. And causes difficulty breathing.
And if it gets worse the upper airways gets more and more swollen and the child also may tend to breathe very fast and might have retractions (when the skin between the ribs pulls in during breathing). In the most serious cases, a child may appear pale or have a bluish tinge around the mouth due to a lack of oxygen.
So... thus, the use of Corticosteroids, in these cases. It is even used with Asthma. I have Asthma. I have used these too.
It is, in acute stages of illness... very.... needed. If not, you cannot breathe or you can die. Which means, it is very very serious and an ER situation.
Your son, if he does not get better and/or the steroid does not help- you NEED to tell the Pediatrician.
Keep an eye on your son... and make sure he is, getting better.