It may be a swollen lymph node because you have a lot of those in the groin region. Those can get really sore and swollen sometimes meaning you have an infection somewhere. If I've been sick, my lymph nodes in my neck will get sore and inflammed.

But wait, you said that it's been there for a year, so scratch that out. From my experience, the swollen nodes with go away after a week or so... Cysts aren't usually painful..

I don't know man, but make sure you get that checked out. Sorry I couldn't be of more help, but good luck!