I'm not sexual active but out of the blue i had a rash grow in my mouth on the side of my cheek. why is that?

Canker sores: Canker sore is the most common oral lesions. Individual ulcers typically last 1-2 weeks. The condition is idiopathic (the cause is not known) recurrent aphthous ulcers is usually self-limiting.

Rash in mouth: This is impossible to answer without a visual and clinical exam, and additional information. You need to see your dentist or an oral surgeon for a thorough clinical exam and medical history.