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.