Erythema multiforme is a particular form of skin rash that arises from an allergic reaction that can happen for a variety of different reasons. These include allergic reactions to viral, bacterial, and fungal infections, sensitivity to food or drugs and reactions to immunisation. It can occur at any age but is often particularly severe in young adults and children. Some people share your experience and often develop the rash after they have developed cold sores. Apart from the rash affected people can often suffer aches and pains and fever. Some people can be quite ill as a result of the reaction. The eruption usually starts as a red rash on the palms and backs of the hands. The skin lesions look like small targets with purple centres surrounded by red rings The rash often spreads to involve the soles of the feet, trunk, face and in severe cases can break out in the mouth. So it is quite likely that the sores you have developed inside your mouth are part of erythema multiforme. I think you should see your doctor in order to have the diagnosis confirmed. Mouth eruptions from erythema multiforme should be medically assessed.