Swolen lymph nodes, whether in the neck, arm pit, or groin, often indicate an infection in the general area. Like tonsils, lymph nodes are "designed" to fight infection. When I was pregnant I developed a cyst in the crease of my leg/genitals. As it got worse the lymph node on that side of my groin became very inflamed. After I had it drained, both it and the swolen lymph node went away. Later, while nursing, I got mastitis and the lymph node in my armpit on that side got swolen.
If it stays swolen and painful for more than a day or two, or you get a fever, get in to your doctor. Otherwise, try no to worry about it too much
Angel, et al