FFS. (sorry had to swear to get it off my chest!)
I think due to terrible weather we have escaped bad hayfever. DS been snotty but as bad as usual the past week so started the cetirizine daily.
This morning VERY lethargic and itchy so gave him 5ml chlorphenamine and off he goes to school - fine all day.
But of course 6 hours after chlorphenamine he starts itching so I leave it thinking he'll have cetirizine in a bit so he'll be OK.
Then the hives appear, on his legs, huge patches of welts bigger than he's ever had before. Then a rash spreads to his stomach but less welty more pinprick. He is itching like a goodun - cue 5ml more of chlophenamine.
Ring to ask GP advice and see if cetirizine as normal an hour later is OK - given all clear and she actually recommended it as a good idea. DS had a cool bath with oilatum.
He is still itching like mad but welts seem less red iyswim?
GP advised him sleeping with me so I can watch him due to his severe reactions always building up previously. Advised if welts spread to neck area even if no breathing difficulties and no swelling A&E to be safe.
So nightwatch with torch and epi-pen on hand for me tonight. 
putting pics of welts on my profile - just because you've all seen them before and perhaps part of me wants someone to say it's not hives!