Thank you so much for your question. It is my privilege to be able to help you.
There are many possible explanations for your pain. Many women complain of worsening constipation with their periods, and certainly it is possible that it is due to stool impaction. This is usually evident on a physical exam, and it is worthwhile to see your doctor to check if this might be the explanation for your symptoms.
It also could be due to endometriosis on the bowel, this pain typically gets worse with your period, and can be associated with bowel movements. Other possibilities include adhesions (scar tissue) in the pelvis, hemorrhoids, or other masses in the pelvis. It may also just be a muscle spasm. It is worthwhile at ;east getting an exam to see if anything can be found that might require treatment.
I hope this was helpful to you. Please remember to ACCEPT this answer if it answered your question so I can get compensated for my work. If you have any more questions please feel free to ask me. Take Care, Dr. SaulJE
It is extremely unlikely that this has anything to do with cancer, for several reasons. Cancer is very very rare at your age, for starters, and cancer is usually not painful in its early stages. However, most importantly, cancer would not last this long, if you have had it for this long, and it was cancer, it would have grown by now and made itself obvious. Furthermore, these symptoms are not typical of any pelvic or gatroenterologic cancer that I am aware of. Still, it is worthwhile seeing your doctor for an exam to try to figure out what it might be. Dr. SaulJE