I'm 22 years old, and I have same problem like yours!
It starts anywhere of your skin, and it's very itchy, and once when you scratch it, it will spreads more to anywhere of your body~
And whenever you don't scratch it, your temperature will keeps going up, and once when your temperature goes higher, it will starts spreads everywhere of your body!
I'm not to sure about this... but there's few things that works on me, that what I've tried before or even now!
1. Stay in any A/C areas; so it will keep down your temperature, to slow down the spreads.
2. If any how you could, take a cold shower; to cold down your body temperature, that could slow down the spreads faster though.
3. Get enough sleep; probably around 6 till 9 hours of sleeps.
4. Don't eat any sea-foods, or any spicy things at all; it will make skin-rush comes out your body easier, whenever your body feels weak (which is you can't feel when will your body be weak or not).
5. Drink "Children's Benadryl: Allergy Diphenhydramine HCI oral solution Antihistamine" (mine it's Cherry falvored ) and if you want, you could use the tiny cup from the NyQuil or DayQuil, and use that tiny cup, and make sure you drink 20mL; it will stop you to feel itchy.
6. If it already spreads anywhere higher than your neck?! Either you could still try those five things above, .... otherwise, go to hospital ASAP!
PS:(Make sure the topic or subject must says "Skin-Rush", otherwise I won't read and will delete it.)