Why does the inside of my vagina hurt?

I'm a virgin and I'm 15 years old. Yes, I do masturbate with my boyfriend on the phone, but I have noticed that lately, the very inside of my vagina, to the left near the opening, it sort of hurts. I'm really confused as to what's wrong with me. The pain is not so excruciating as to where I can't do it, but I can slightly feel the pain. Can some one help me maybe?