Hives after an antibiotic or something else?

Kellan was on amoxicillin from Jan 29-Feb 8. Yesterday he woke up from his nap with hives

Now, I chalked it up to the amoxicillin or the cold he was getting over(pneumonia) because his brother was the same after being on amoxicillin the first time. So last night I was going to give him an oatmeal bath but it ended up going away on its own before that happened.
This morning was fine, no hives or rash, he does have a bit of a teething rash around his mouth from all the drool.
He took a nap and now they're back and worse. Its all over his body and the back of his head. 2 little hives on his cheek though. But the ones on his body look like this


I'm stumped. Why are they showing up after his nap? He hasnt had any new foods, we've been keeping him on a brat diet after the amoxicillin gave him diarrhea and hes used to all those foods. No new soaps in his bath or to wash the clothes.

I was trying to do a bit of my own research online and most posters are saying this would have to be his second time taking the amoxicillin for it to be a reaction to that. So what could it be? I'm so confused. He isnt showing any discomfort and his breathing is fine, that was my first concern because I will take him to the ER if that seems to become difficult for him.

Any ideas? Been here before?

I don't think it's from the medicine. Hives can show up out of nowhere from anything really. I just had a outbreak of them a few days ago still dont know why. Just keep a eye on it they might go away soon atleast mine did. Is he trying to itch it?

Quoting κατι:" I don't think it's from the medicine. Hives can show up out of nowhere from anything really. I just had ... [snip!] ... had a outbreak of them a few days ago. Just keep a eye on it they might go away soon atleast mine did. Is he trying to itch it?"

No, hes still happy and showing no signs of discomfort.

My son had hives just like that when he was an infant and his pedi said it HAD to have been something he ingested (ate) for hives to show up like that

Quoting Koltons*Momma*6wks*:" My son had hives just like that when he was an infant and his pedi said it HAD to have been something he ingested (ate) for hives to show up like that "

I'm going to go through everything I've fed him I hope hes not developing an allergy to the things he's been known to love  

Quoting Sarah Bare-ah♡:" I'm going to go through everything I've fed him I hope hes not developing an allergy to the things he's been known to love  "

I never did figure out what gave my son hives :/ he hasnt gotten em since and hes 2 1/2

Quoting Koltons*Momma*6wks*:" I never did figure out what gave my son hives :/ he hasnt gotten em since and hes 2 1/2"

Now I'm brainstorming and I did give him pumpkin puree a few days ago but only the 1 day. I wonder if that was it...but he would have had to have it twice for it to be a reaction based off what I'm reading.

I have EXACTLY the same hives. I've had them for years and have no real idea what is causing them BUT I did notice a few things:
1) I always had them after antibiotics or when my immune system was low, like after a cold
2) I would always get them during night / sleep time, I think cause the fabric/friction + warmth made them worse, if that makes sense
3) I long suspected it had to do with something I ate, but I was never sure untill the hyves progressed into my lips swelling up beyond control (they even split and burts), and me needing anti histamine to get the swelling down. But it never showed up consistently after a certain food so I still don't know.

Sorry I'm no help, but if you do find out what causes it please tell me!!