Smooth red plaque on skin, like a hive, it blanches when pressed and is half a centimetre long, about 4mm wide. its 1-3 months old. it seems to have appeared sudden. not growing in shape but noticed maybe more swollen looking. could it be cancerous?

Many things are possible in life but skin cancer in a teen is almost too rare to worry about. That said, there are a few teens with skin cancer somewhere in the country. A spot that swells and itches a bit like a hive or bug bite, but swells/itches for no apparent reason, might be a mole-like spot that contains "mast cells" that release histamine. Dermatologist can look at it in-person or online.