Side Effects of Steroid Shot for Croup?

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.

There are many, many different steroids.
Anabolic steroids are the ones that can get abused by athletes.
Corticosteroids are the ones used to treat inflammation.
Long term use of steroids can be a problem but when it's used to treat an inflammation, it's very short term.
I don't get poison ivy often but when I do the reaction will not shut down on it's own (my record for scratching was just over 2 months) and a Prednzone series (steroid) is the only thing that stops it.
When my son was 3, he was on the same thing for 2 weeks to try to shrink his tonsils (worked while he was taking it, but they blew back up when we finished, so we had them out).
Not being able to breathe is way worse than the little dose of steroid that is used to help clear it up.

I call it "devil medicine", because it turns children into maniacs. I hope he is well soon and make sure he is really sick enough to need it if it comes up again.

Your child struggling to breath or a steroid shot? Is there a choice?

Yes, the hyperactivity is definitely a side effect of the steroid used to treat croup. My older son also had croup as a baby - every spring and fall for 3 years he got it. Only once it was bad enough to need a shot. The other times I took him in to the office as soon as I heard the tell-tale cough and got an oral steroid for him instead.

The short term hyperactivity effect with both the shot and oral version) is awful, but this type of steroid doesn't have long term side effects. The type of steroid that people get in trouble for using is a completely different type than the one used for croup. Totally and completely different drug.

The good news is that since your son is 3, he should outgrow croup soon.

My kids have had them (for croup) and they are fine. I decided to do it for my kids because they were struggling to breathe and were so tired. My oldest tends to vomit when he has croup, and it was no fun for anyone, especially him.

I'm crossing fingers for a croup free year this year, my kids are older now so I'm hoping the worst is over.