I had the exact same thing for about two years! I went for scans and ultra sounds and they confirmed with me that it was just a type of hormonal cyst. I tried different tablets to bring down the swelling as it was very deep under the skin and wouldn’t really come to a head for me to try to burst it myself and nothing really worked, eventually my doctor offered to cut it and try to drain it as I couldn’t move freely without being in pain. After she cut it filled up for about 3-4 months afterward as I approached my period but it gradually fizzled away. I did a bit of research online and it’s a sign that your ovaries are functioning as normal, I am on the contraceptive implant and don’t always get a period every month but i still go through my cycle and this is just a thing to remind me that my ovaries are working the way they should!