Predisolone for new skin allergy in 7yr old?

[AllergyWorry]

Hello wise ones.

My DC age 7 woke up yesterday slightly covered in hives. We gave piriton to no real improvement and changed all bedding.

No new powders etc or anything new has been used (by way of background)
As they hadn't improved today hives spread to face we took to OOH who have said 'some allergy' and given 20ml pred.

My questions-

What happens next? I'm so worried but they just said ring GP. How does an allergy just pop up when nothing has changed and does this mean it will get worse ( I guess no one can tell but I'm so worried)

Is this level of pred high?

Thank you so much for any replies.

My kids got hives as an immune system overreaction to a minor virus as kids; this is more common than a sudden allergy. Could it be this?

https://www.seattlechildrens.org/conditions/a-z/hives/#:~:text=Hives%20All%20Over%20the%20Body%20%2D%20What%20You%20Should%20Know%3A,%2C%20drug%2C%20or%20insect%20bite.

Hives are not intrinsically scary. I wouldn't do pred unless they were very distressed by the hives.

Thank you so much.
DC has had a very slight cough recently and one episode of upset tummy so I'm thinking it may be linked now?
I will have a read of that link.

https://www.texaschildrens.org/content/wellness/hives-hives-and-more-hives

Here's another link

I think hives accompanied by lip swelling, mouth swelling, feeling of tight throat or chest, difficulty breathing etc are a different kettle of fish and obviously would be signalling an anaphylaxis type response to an environmental trigger. My kids when they had frisky immune system response had none of these symptoms, just patches of hives that circulated around their bodies over a few days.

Thank you for taking the time to post this useful information. Much appreciated.
The hospital gave DC one lot of oral steroids directly and now hours later the hives have all but gone. We have more to be used if needed and hope they won't be. I'm so relieved and hope this is a one off.

You can develop an allergy at any age

My auntie developed a nut allergy in her early 20s. I developed a histamine intolerance at 21 and had 2 anaphylaxis reactions before diagnosis.
In my mid 20s I was tested for liver disease as I couldn't stop itching but with no rash and it turned out I had become sensitive to washing pods, shower gel and shampoo/conditioner. I kept the same products for years. I switched to washing powder and solid soap and shampoo/conditioner and all the itching stopped. I realised I had always been prone to itching but sporadically and it went completely with the changes.

I can't remember my dose of prednisolone after having my anaphylaxis but they were 5mg tablets and I had to take something like 10-12 tablets at a time. I was an adult but 20mg doesn't sound a huge dose. It's probably worth a follow up with the GP next week and get an referral to someone specialising in allergies for some testing.

Thank you @Superscientist
Those are all very thought provoking points.