I've had a cold; but I have this small, painless bump on the corner of my mouth. I have been tested, I'm STD free. Doesn't look like blister not red?

Angular cheilitis. Is inflammation of one or both, of the corners of the mouth. The condition is a bacterial or fungal infection. Stress and weak immune system are a contributory factors. If untreated will take a several weeks to heal. Rec.: see you pcp for the diagnosis and treatment.