If I were you, I'd make an appointment with a gynecologist.
However, you should not confuse difficulty with your menstrual cycle (at age 21) with possible infertility.
Also, do not confuse having difficulty with painful intercourse with any other more troublesome issues. Frankly, it might be any of several benign things including lubrication, being new at having intercourse or a need to change positions or angles of entry.
You clearly have quite a few questions for a gynecologist and seeking such medical expertise from a gyno can only help you.