My first suspicion is a hip problem however is the groin pain the ONLY pain you are experiencing? Do you have any type of mild lower back or pain/stiffness anywhere else? Do you have any discomfort when sitting for long periods of time, squatting, or when trying to sleep?
My other guess is that you may have knocked your pelvis out of alignment on the side where you have groin pain. Because your pelvis is obviously 3D, this could happen in any of the 3 planes of motion. For example, one common rotational problem is an anterior rotation of the Ilium along with a nutation of the sacrum. This can cause a lot of chronic muscle tension (causing general ache/pain) usually around the groin. This is because since your pelvis is hiked up on one side the adductors remain tight and shortened which will start to cause general aching and even sharper pains if you were to palpate them under tension. The more activity you do that involves side-to-side movements, sitting, squatting, etc the more inflamed and tight the area will get. With rest it will die down and almost feel normal. But nothing has been done to correct the actual pelvic imbalance so once you go back to activity, BOOM it hurts again.
I would definitely keep a possible joint issue in mind (particularly hip impingement). But I would also visit someone who is knowledgeable with pelvic alignment, particularly at the SI joint (where your sacrum and ilium meet). They should check your spinous processes in the front and back to make sure you are properly aligned.