This is very likely a Bartholin gland cyst, and the cause could be your menstrual cup OR sex. Vaginas have two little glands close to the vaginal opening, and their purpose is to release good, healthy mucus that lubricates your vagina and the vulva. However, sometimes these glands get plugged up, which can just happen naturally OR happen when there is irritation or trauma to the area (like penetrative sex or use of a menstrual cup). The result is the feeling of a little ball or nodule at the opening of the vagina.

To help unplug your glands, try soaking in a warm bath a couple of times a day. If the gland(s) starts to hurt, interferes with sex or inserting your menstrual cup, causes discomfort or starts leaking pus you should see a doctor (because it might be infected).