Painful sex?

Sex with my girlfriend is always extremely painful and it's frustrating for both of us. I haven't told her how much it hurts but instead I'll fake an orgasm or tell her that I'm too tired to keep going Tampons are also painful but not to extent of sex. It's a burning pain that feels like a hot iron has been shoved inside of me. Anyone know what the cause may be or what I should do? Or if this is normal and I'm overreacting