Hi,
What you've described isn't uncommon; unfortunately many women have dealt with similar problems that are much like yours. There's a long list of possible causes, such as: extremely deep penetration that is causing pain, possibly a retroverted or tilted uterus, cervical issues, cysts, or Endometriosis, etc... You really should consult your doctor or gynecologist about this; you may need an ultrasound to help pin-point the cause. Meanwhile, you might want to try different positions during intimacy to see if it helps ease the pain, and an anti inflammatory pain med.(like ibuprofen) afterward.
Take Care