I would get an opinion from a Gyn/Onc. They are more experienced and even if you don't have cancer, they may give you some very good advise. As I say you're special so why not see a specialist. There are many Gyn/Onc that perform surgery on women that don't have cancer. Having these many periods during the month with pain is no good.
Please don't freak out that you have cancer but do yourself a big favor and get an opinion from a good Gyn/Onc.
Good luck.
It appears that the mass (tumor or cyst - all words are interchangable) has shrunk which is always positive and it being simple is also another a good sign. Could what you are feeling be side effects from the Pill that you are on????
You might have cancer, but, I doubt it. OvCa is rare and even rarer at your age (although it does and can happen)...most OvCa cysts are benign, well over 90 percent.
The doctor seems to be following what is common protocol...come back in a couple of months to see what is happening. I would personally fell more comfortable with the response from my GYN regarding my ovaries vs my internist. If you are not happy with the response from the GYN, then maybe think about a 2nd opinion. It is common to have women return for additional scans after a cyst has been discovered.
Good luck and keep us posted.