An inguinal hernia is a weak spot in the abdominal wall, on either side of the groin, just above the leg/abdominal crease. If you bear down (like grunting) and can feel something pop in and out, it's very probably a hernia. I had a minor one develop spontaneously about seven years ago. It's relatively minor surgery, but don't plan on driving (or doing anything really) for about a week. When in doubt, see a doctor. And, if it is a hernia, PM me for helpful hints during your recovery.
Hoping it's just a pulled muscle. Hang in there.