If she is worried about cancer, this surely isn't cancer at all, tell her not to worry.
As for pain, there are multiple causes, including PMS (Pre menstrual syndrome), hormonal changes, minor infections etc. One would need to assess her and understand her complaints to give some medicines.
Ask her to take pain killers for a day. And if it doesnt settle, preferably just show to a doc. It will surely settle down, but its essential to take the right advice hence to see a doc.
As doctor Shah said, your friend's pain is probably nothing, but having it checked will give her peace of mind. When I was in my early 20s I had a large lump appear in one breast. I was sure that it was breast cancer and that it was going to kill me, but also did not want to worry my parents. It turned out to be a fluid filled cyst. My doctor drained it and that was the end of it.
I did tell my mother. I hope your friend can talk with her mother, also. It could be that she had the same issue when younger, and could share that with your friend. Meanwhile, she is lucky to have your support.
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