Myositis Ossificans: This condition is a common complication of muscle injuries, particularly in the thigh and hip region. It occurs when there is heterotopic ossification of the muscle, leading to the formation of a bony mass within the muscle tissue. The presentation of a small, hard, painful, and immovable lump in the pelvic psoas region a few weeks after a groin injury is consistent with myositis ossificans.

Hematoma: A collection of blood outside of blood vessels, which can occur after a muscle injury. Over time, the hematoma can become organized and form a palpable mass.
Muscle Strain with Fibrosis: Repeated strain or injury to the muscle can lead to the formation of fibrotic tissue, which can be palpable as a hard lump.
Bursitis: Inflammation of the bursae, which are fluid-filled sacs that cushion joints and reduce friction between bone and soft tissue. Bursitis in the pelvic region can cause pain and swelling, which may be mistaken for a lump.

Testicular Torsion or Epididymitis: Although less likely, these conditions can present with pain in the groin region and should be considered, especially if there is a history of trauma or infection.
Osteosarcoma or Other Malignancies: Although rare, it is essential to consider the possibility of a malignant tumor, especially if the lump is growing or if there are systemic symptoms such as weight loss or fever.
Intra-Abdominal Pathology (e.g., Appendicitis, Diverticulitis): In some cases, intra-abdominal conditions can present with pain and a palpable mass in the pelvic region.

Avulsion Fracture: A rare condition where a fragment of bone is pulled away from the main bone by a tendon or ligament.
Psoas Abscess: A rare condition where an abscess forms in the psoas muscle, often as a result of a bacterial infection.
Desmoid Tumor: A rare, benign tumor that can occur in the abdominal wall or pelvic region, which can present as a hard, painful lump.