Patellofemoral Pain Syndrome (PFPS): This condition is characterized by pain around or behind the patella, often associated with activities that put stress on the knee, such as running, jumping, and climbing stairs. The recent increase in running and rucking activity without a specific injury makes PFPS a likely diagnosis due to overuse and potential biomechanical issues.

Iliotibial (IT) Band Syndrome: Friction and inflammation of the iliotibial band as it moves over the lateral femoral condyle can cause knee pain, especially in runners and individuals who have recently increased their activity level. Pain is typically worse with downhill running or walking and may be exacerbated by stairs.
Medial Meniscal Tear or Strain: Although less common without a specific injury, the increased stress on the knee from new activities could potentially lead to a meniscal strain or a small tear, especially if there are underlying biomechanical issues or previous unnoticed trauma.
Quadriceps or Patellar Tendinopathy: Overuse from increased running and rucking can lead to tendon inflammation and pain, either in the quadriceps tendon or the patellar tendon, both of which could cause knee pain that worsens with stair climbing.

Osteonecrosis of the Knee: A condition where the bone beneath the cartilage of the knee dies due to lack of blood supply. It can cause severe knee pain and is more common in individuals with certain risk factors, but it's crucial to consider due to its potential for significant morbidity if not addressed.
Spontaneous Osteochondrosis of the Knee (SONK): A rare condition but important to consider, especially if the pain is localized to a specific area of the knee and worsens over time.
Infection or Septic Arthritis: Although unlikely without other symptoms such as fever, redness, or swelling, it's critical to rule out an infectious cause, as septic arthritis can lead to rapid joint destruction if not promptly treated.

Osgood-Schlatter Disease: Typically seen in adolescents, this condition involves inflammation of the patellar tendon at its insertion on the tibia, causing knee pain and swelling. It's less likely in adults but could be considered in younger individuals.
Sinding-Larsen-Johansson Syndrome: Similar to Osgood-Schlatter disease but involves the patellar tendon at its origin on the patella. It's rare and usually affects adolescents.
Chronic Regional Pain Syndrome (CRPS): A condition characterized by persistent burning pain and hypersensitivity after an injury or trauma, which could potentially follow overuse or minor trauma in the context of increased physical activity.