What can be the causes for pelvic pain?

Many. Infection, Inflammation, Masses, Growths, Scar Tissue, Muscle Spasms, Nerve Disorder, Back Disorders, Medical Conditions.....There are so many causes. I recommend further evaluation and testing. Check out pelvicpain. Org.
Need more detail. There are many causes of pelvic pain, from skin, muscle, and bone/joint problems, to gastrointestinal or reproductive/urinary issues. A full history and physical examination by an experienced physician is usually very helpful.
Long list. There are many possible answers. Appendicitis, ovarian cyst, pelvic inflammatory disease, interstitial cystitis are possible answers.