Hi, a red bump that your describing is not a symtpom of any std esp from an affair that you had a year ago. Since it comes and goes it maybe natural to your skin and reflecting back to the affair maybe anxiety related. Since the tip of the penis is full of blood and any pressure could force it to the surface, it would be more related to something natural.
the burning is almost certainly from you constantly opeing the tip looking for things and the color is from blood reflection as is foll of blood vessels in that area.
If you had an infection you would have a white puss discharge as well.
Not from sometiing a year ago and maybe its something you caused by masturbation.