He is 3 and has been ill with a bad cold for which he now has antibiotics. The last 3 nights he has come out in hives when he sleeps. They are bothering him and he wakes up crying. They calm down in the day.
There has been no new washing powder or anything so can't work out what is happening.
It could be the antibiotics - you should check that as that's serious.
But it might be stuff in the air that gets on his sheets. There is a time of year where I can't dry my son's sheets outside or he'll wake up with a swollen face. I find if I tumble dry them (not just dry them indoors), it doesn't happen.
When you say the rash 'calms down' in the day, does it completely go, or is still visible but not bothering him...? Could it be a post-viral rash? My DS (who is mildly asthmatic, but with no particular significant allergies) always get patches of blothcy spots, especially on his chest, back and legs after any and all viruses. The tend to appear 7-10 days after the original bug starts and can be very itchy - enough for him to draw blood during the night, but the itchiness increases at night and they are less of a bother (though still visible) in the daytime. MPD
Could be heat actually, he has this toy stuffed lion that he insists on sleeping on but it makes him really hot. I have moved it away now that he is asleep so hopefully that might help. Also washed his security blanket just in case.