Muscle aches in the groin or the abdomen are very common. As you have mentioned, they are more common in people who have recently been engaging in vigorous physical activity or who have had an injury. However, it is possible to accidentally pull something, even with just minor activity. If you have been doing any sit-ups, stretching, jogging, etc that is not a typical activity for you, this is likely the cause. If so, you just need to wait it out a few days, and things should get better. You can apply ice to the area for relief and also consider taking an over the counter anti inflammatory medication like ibuprofen.
Another common cause of abdominal and groin aches, especially in men, is an undiagnosed inguinal hernia
, which occurs when contents of the abdomen bulge downward towards the scrotum. This is a condition that can go on for a long time without diagnosis, as the findings can be pretty subtle. Your primary care doctor
, however, will be able to diagnose the condition right away. If you do have hernias, you will need to be referred to a surgeon
to discuss your options for surgical repair of the hernia.