I would like to ask for your opinion.

I’m feeling tightness in the groin, specifically the adductor muscles.
Both sides tighten up, but more so on the right. It’s just one area that feels tight, and I feel tension there. I haven’t strained anything or had any injuries.

I stretch gently, but it doesn’t help at all. I’m quite active; I run, exercise, do leg and abdominal strength exercises, but this muscle can get very tense. I don’t know what else to do anymore.

There are four common causes of groin pain in physically active individuals – muscle injury (strain or partial rupture), tendon overuse syndrome of the groin muscles (tendinosis, enthesopathy, depending on whether the process is in the tendon itself or at its attachment to the bone), groin hernia (or colloquially, hernia), and hip impingement or arthritis.

From this brief description of symptoms, it’s only possible to conclude that it’s not a muscle injury because those typically have a clear and sudden onset, usually during a sudden movement or fall, which is not the case here.
It’s possible that it’s a condition related to the tendon or its attachment, but I would expect some reduction in physical activity and persistent stretching to be helpful.

Groin hernia (or hernia) may not be visible as a bulge. Even minor weakness in the abdominal wall can cause pain during more intense physical activity. It often comes with pain when coughing or sneezing.

Hip impingement is a condition where, due to specific anatomy, some movements are restricted, leading to the development of pain, primarily located in the groin region.
Which of these diagnoses is correct in the specific case can only be determined through a detailed examination, and the therapy radically differs for each of them. The shortest path to diagnosis starts with a simple X-ray of the pelvis and hips, followed by a physical examination, and possibly, if requested by an experienced orthopedic surgeon or physiotherapist, an ultrasound or MRI.

As always, I do not recommend self-diagnosis or treatment based on internet instructions or advice from well-meaning but unqualified acquaintances. It’s not uncommon for such actions to delay appropriate treatment, thereby prolonging it, sometimes significantly.