What Causes A Swollen Groin Lymph Node?

I am a 39 year old female. A few days ago I found a small (size of a marble) swollen groin lymph node on the right side. Never had this before. It is small, hard and tender to the touch.
I was sick last week with a sinus infection, butI don't think that's causing it.
I also have an ingrown hair fairly close to it, but I've had those before without a swollen node.