Hi dear you are either suffering from dermatitis
which is probably due to the consumption of food you are allergic to which is causing rashes on your body as well as on inside of your mouth. Most of the rashes subside by themselves and do not require any medication but they can recur with the consumption of the same food.
Another reason can be you have chickenpox
if you never had it before. The most common symptom is itchy blisters
over the entire body which can effect the mucous membranes too alongwith other symptoms like bodyaches, fever
, irritability. These blisters lasts for 5-7 days and heal with scabs.