strange rash on stomach

hi
i was just looking for info on a rash that appears on my stomach after i have been lying down on it {my stomach side}sometimes when reading or on the pc i might lay on my stomach side for comfort and after about 10 mins i have to turn cause my whole stomach area is covered ina rash .it is reddish and stings and will go away after about 20 mins .
is this some sort of heat rash or contact dermo or something ,it can happen on my back also when lying against a pillow and can be quite un-comfortable ,it is very confusing,
any thoughts would be welcome

[Dr_Aparna]

Hi,
Hives is an allergic skin reaction causing localized redness, swelling, and itching.Hives is a reaction of the body's immune system that causes areas of the skin to swell, itch, and become reddened (wheals). When the reaction is limited to small areas of the skin, it is called "urticaria."
Hives is an allergic reaction.When mast cells encounter an allergen, they release histamine and other chemicals, both locally and into the bloodstream. These chemicals cause blood vessels to become more porous, allowing fluid to accumulate in tissue and leading to the swollen and reddish appearance of hives. Some of the chemicals released sensitize pain nerve endings, causing the affected area to become itchy and sensitive.
Sometimes pressure /heat can also cause it, which would be what is happening with you.
Mild cases of hives are treated with antihistamines, such as diphenhydramine (Benadryl). More severe cases may require oral corticosteroids, such as prednisone.
They usually disappear on their own once the stimulus is removed.
ref:http://www.myonlinewellness.com/topic/hives

[lagoya]

thank you for the very imformative answer