It didn't swell from an injury, it's been there for at least a few years. But it never really increased or decreased in size.
It sounds like you're describing a lymph node. These little sacs are part of your body's lymphatic system that carries waste away. They're found all over the body but are most noticeable in your armpits and groin.
Normally you don't feel them (unless you're a health professional and know where to look). However, any injury or infection can cause them to swell up because they're carrying more waste. If the ones in your groin swell it can be from something as simple as a cut on your leg on that side. You might remember your mother feeling your neck when you had a sore throat to see if you had "swollen glands." She was feeling for swollen lymph nodes as a sign of infection. (they're not glands at all, but that's what lay people call them).
If that's wht it is it should go away in a week or so and return to normal size. If not, it's something you should point out to your doctor on your next visit.