Expert Q & A Answers About whitehead

How do I know if I have a pimple on my penis or if I have an STD?

The other day when I was urinating, I felt a little bump on my penis. I squeezed it and a whitehead came out...just like a pimple. Since then, the "pimple" got bigger and irritated. It’s the first time it has happened to me...should I be worried?