Hello
I read this last night and have been meaning to post a reply all day and am only just getting around to it - sorry.
Right, my DD has multiple food allergies and at Christmas 2010 she started to get the same sort of hives/ rash that you describe. She seemed to get it after eating even though she was eating food that I thought were 'safe'. She also seemed to get more hives/redness/rashes on her face when she was sleeping, on the side she was sleeping on. I gave her piriton and panicked like hell as I was so so scared that she was suddenly reacting to 'safe' foods. After a very upsetting week and loads of piriton, I finally managed to get through to her allergist. By this stage the piriton had stopped working, ie it wasn'ts ettling the rash, and her lips had also started to swell up, so I was seriously freaking out.
Because it was worse when she slept, I slept on the floor of her room and made her sleep under a sheet, as I thought the heat of her duvet was making it worse.
The first time her lips puffed up I took her to A&E and it was odd as by the time we got there it had settled. I wasn't sure if it was the effect of the piriton. Whilst we waited for couple of hours to be seen, the rash and swelling came up again and then settled and came back again. The rash also seemed to move. A&E told me it was an allergic reaction and to go home and keep giving the piriton. I remember crying and saying 'but it's not working'
The odd thing about this episode was that the rash did come and go which I hadn't ever really seen with her other reactions before.
Anyway, the allergist asked me (when we finally spoke) if she'd had a cough or a cold. DD had had a cough a few weeks earlier but nothing to write home about and it seemed to clear up by itself.
The allergist explained that it wasn't an allergic reaction. He said that when the body produces antibodies to a virus, sometimes the mast cells under the skin that realease histamine which causes the rednes and hives, react to these. This causes histamine to be released and therefore you see hives, redness etc. I asked him about the lip swelling and he said that was the same sort of thing and nothing to worry about. He said that this is called Virally induced urticaria and that Piriton isn't as good for it as it's not that long lasting. He told us to give DD a non sedating antihistamine. We had neoclarityn so he said that was great. This was good as DD gets very tired on piriton. He also told us that this could take a few weeks to get over and to keep her topped up for at least 2 weeks. In the end it took 3.5 weeks before I felt that she didn't need it. I reduced the dose to once a day, at night, and she was definitely not having any more hives appear after a few days so I cut down to a half spoon and then none. The fcat that she had hives and redness on the side of her face that she slept on was because pressure can also aggravate the mast cells so I needn't have made DD sleep under a sheet and it made sense that if she turned over, the redness and hives would go after a while.
I asked him how he knew she wasn't reacting to her foods as she seemed to get a flare up after eating and he said that as the rash was reacting to pressure and moving about, it sounded like it was due to fact that she's had a virus.
He said that they get alot of referrals from A&E where the doctors fail to check for a virus. Often, he said, they see people with no history of allergies and when he sees them in clinic there are no signs of allergic reactions during their tests.
He also said that this virally induced urticaria can affect anyone and not just those with allergies.
SOrry for the long post, but I hope this helps. Definitely ask to be referred for testing but as the rash was on the move, it sounds very much like what my DD had and although she has allergies, it wasn't actually an allergic reaction that time. DD has had this once again after that episode, and I knew what it was because the rash moved. Also, if you find the rash is still there after 24 - 48 hours of taking the piriton, then maybe it might be worth asking your GP for a longer lasting antihistamine
Take care and sorry for any silly typos.