You say that it hurts on the outside but the bump is actually located on the inside?
Is the bump on the inside of the Labia majora or actually inside of the vaginal opening?
Is the bump a different color than the other skin around it?
Does it have a head like a pimple?
How large is it (large enough to block the opening, pea sized, etc.)?
Answering these questions can help me to help you better ! : )
I would suspect because of the location that this is related to either a swollen lymph node, cyst, abscess or duct obstruction. This can be common in this area and no cause for concern unless it is growing, causing you great pain or doesn't go away or appear to get smaller within a week or so. If the lump appears to be getting bigger, or starts draining discharge you will want to have it checked right away because this is when a blockage can have progressed with an infection.
If you have not had a GYN exam with pap smear in the last year, I would strongly suggest that you schedule an appointment. This is very important for detecting cancers at early stages when they are most treatable and can greatly improve your health and well being.
There is a small possibility that this can be related to a very common infection called the Human Papilloma virus (genital warts). Though the lump you are describing does not necessarily sound like a genital wart, these can present very differently. I will provide a link to you for more information, but your HCP would need to visually diagnose this if it were the case. http://www.cdc.gov/std/hpv/
I hope this is helpful to you; please remember to press ACCEPT and feel free to contact me if you have anything to add!