This is a good question. It is very important for you to make an appointment with your primary care doctor
or ob/gyn to discuss your symptoms and have a complete history and physical. Depending on what your doctor discovers during your appointment, he or she may perform testing as well. A bump on the vagina that comes and goes could be many things. It is possible that you have a Bartholin's cyst, which is a fluid filled collection that occurs when the Bartholin's ducts (the tubes that lead from the glands to the vulva) becomes blocked. These can vary in size somewhat but is also possible that you are just more aware of it during your period if you are inserting tampons and such. Another possibility is herpes, or HSV2, which is an STD
. Many people experience outbreaks during their menstrual cycle. These usually present as bumps which then turn into painful blisters. If you are sexually active make sure to mention this to your doctor to see if you need tested for this or other STDs. Again, it is very important to make an appointment with your doctor to discuss your symptoms and get an appropriate diagnosis and correct treatment.