I am 15 years old and about a year ago I noticed small yellowish bumps on the shaft of my penis. I have since discovered what these were, but recently I have noticed a bigger spot underneath shaft of my penis. It is about 2-3 mm and it appears as if there is a slight swelling around it. I am worried what is it.
I would be willing to research the possibilities, but you haven't given me enough information with which to work. What color is this bigger spot? Is it tender to the touch? Does it itch? If you look very closely at it, is there a hair follicle in the center of it? You have to remember that all I have to work with is exactly what you tell me.
Yellow colored bumps generally point to the oil glands (called the sebaceous glands by doctors). They are called "Fordyce spots." You can see them more easily on the shaft of your penis than elsewhere on your body because the skin is very thin on your penis. Fordyce spots cause no harm.
The only certain way to tell what you are seeing is to have a doctor who specializes in the skin (dermatologists) take a look at it.