The differential diagnosis of pain in the left lower quadrant of the abdomen or groin is quite extensive, which is why questions on history-taking and a clinical examination are so important. I see some investigations (MRI) have been completed, which should narrow it down a bit.
This diagram shows the position in question: the left lower quadrant (or left iliac fossa) and the left groin region.
This anatomical image shows the various deep structures in this area (though the image focuses more on hernia repair).
This is a very useful summary of the potential causes and has useful links itself. Using this as a guide, I will go through the possibilities.
Firstly, the pain is chronic rather than acute because it has gone on for a year now. This helps us exclude short term causes like urinary infections, diverticulitis and gastroenteritis.
Constipation: Chrinic constipation is often due to diet and the pain is usually left sided or central lower abdomen.
Irritable bowel syndrome: This can cause a variety of symptoms, including cramps, bloating, nausea, constipation and loose stools. This is a diagnosis of exclusion, meaning that it is important to exclude other causes before diagnosing this, as there are no confirmatory tests.
Cancer of the rectum or descending colon: This would usually cause a change in bowel habit, weight loss and bleeding. The MRI scan would probably have excluded this, though colonoscopy would be useful.
Crohn's disease and ulcerative colitis: These are types of inflammatory bowel disease. They would usually cause a change in bowel habit as well.
If the person is female, possible causes include pelvic or ovarian tumours and endometriosis. The MRI may have excluded the former, but laparoscopy is often needed to diagnose endometriosis.
Left hip pathology: This could be arthritis or other mechanical problems with the soft tissues around the hip and upper leg. The fact that the pain changes with position might suggest one of these causes. MRI scans might miss muscular causes, but would detect arthritis. Sometimes nerves can be entrapped which will give the pain a different character.
Image of pelvis and hips from Radiopaedia.
Hernia: Some groin and femoral hernias might be missed on MRI scanning if they are small or reducible.
Medically unexplained symptoms: Sometimes a purely physical diagnosis is not forthcoming. Sometimes this reflects the limits of diagnostic and imaging technology and sometimes it results from the fact that the human body is amazing and the interaction of mind, nervous system and body is complex. Sometimes this complexity gives rise to symptoms that doctors can’t explain.
The positional nature of the pain and (presumed) lack of bowel or urinary symptoms would probably make a musculoskeletal cause for the pain most likely, or possibly a hernia. Given that it has been there for a year and there have been some normal investigations, it is unlikely to be a serious or progressive problem. However, other investigations may be needed to find the cause more definitively. Please note that this answer is not providing medical advice. It is important to see a doctor or healthcare professional if you are suffering from these symptoms.