I would recommend that you go to see your OB GYN doctor
or your primary care doctor
about this issue. There a couple of different potentially causes here, but a doctor is going to have to take a look at the bump to figure things out completely.
One possibility is that this is an ingrown hair. Sometimes these can develop a fair amount of swelling around them and be painful. They are also more common in people who have a history of recent pubic hair shaving, but they can also occur without shaving. Most of the time the don't require specific treatment other than warm compresses and good skin hygiene.
Another possibility is that you have developed an abscess
under the skin. This could have started as an ingrown hair, or it could have been an unrelated skin infection. Many times abscesses need to be opened and drained in order to heal properly, and your doctor will be able to figure this out.
Finally another possibility is a Bartholin cyst, which occurs when one of the normal glands around the vaginal opening becomes blocked and swells up.
Please call and make an appointment with your doctor to get this sorted out as soon as possible!