Sounds like a swollen lymph node. The inguinal lymph nodes are located in the groin area. Lymph nodes usually swell up when there is an infection in that area. Are you having any symptoms of an infection?
I would talk to your general practice physician. They might have an idea of what's causing it to swell. If he thinks it's due to some sort of infection in the vaginal area, then you should see the gyno to get it checked out. It sounds to me like there is an infection somewhere. You could just be asymptomatic, which isn't rare. For example there are women that get bacterial vaginosis and never experience a symptom (in case you don't know, BV is a VERY common infection that occurs when the vaginal pH gets off balance and allows an overgrowth of "bad" bacteria). Anyway, you should get checked out to rule out any type of infection. If left untreated, infections like BV can lead to pelvic inflammatory disease, scarring, etc. Best wishes to you! Keep me posted on how it goes!
BTW...one other thing I forgot to mention....I had a swollen inguinal gland one time and came to find out I had a vaginal infection. That's why I'm harping on the infection thing!!!! And keep in mind, I'm using the scenario of a vaginal infection, but it could be an infection anywhere. It's just that usually infections cause the lymph nodes in that same area to swell. Another example, my mom found a lump in her breast and at the same time the lymph node in her armpit was swollen.
Years ago I had a swollen lymph node there, also, due to bacterial vaginitis. I went to my gynecologist at the time and was treated for it and have never had a problem since. So check with your primary care physician or gyn and you should be fine.