These rashes sound quite bothersome to you. From what you have described, they seem to be hives, also known as urticaria. They will suddenly appear, be very itchy, sometimes associated with a burning sensation, and then disappear in a few hours, usually within the same day. Please try your best not to scratch them as the rashes may worsen with excessive scratching.
There may be a trigger at times shortly before the onset of these rashes. The most common triggers are a recent infection (cough/ flu/ fever), allergy to a certain medication or food. However in most situations, patients are unable to recall any particular trigger.
Hives can be unbearably itchy, and anti histamines can help to alleviate the itch. If the hives are recur on a regular basis, it is best to seek a dermatologist's help as blood tests may be needed.
If there is difficulty breathing, funny wheezing noises from the throat, swollen eyelids or lips when hives occur, you will need to go to the emergency department as soon as possible as it means that your airway may be blocked.
I hope that your condition improves soon, take care!
Dr Ker Khor Jia