Hi and welcome to the forum! I am glad that I can help you.
You want to know the cause of the symptoms that you experience.
Pelvic inflammatory disease usually appears as a result of bacterial infection of the reproductive organs. Inflammation of the uterus or fallopian tubes may spread in the nearest pelvic tissue. The inflammation may be localized and may persist as an abscess located between the utherus and rectum, called cavum Douglasi. It is characterized by pain in the pelvis.
Given the data that you described, you may or may not experience pelvic inflammatory disease or an abscess in the cavum Douglasi. See a gynecologist for further diagnosis. You may need to take antibiotics.
Please keep in mind that I provide medical information only. I am not able to diagnose medical conditions online. Please contact your doctor or other qualified healthcare provider such as a gynecologist for further advice and information about diagnosis and treatment options.