Biliary Colic: This condition is characterized by intermittent, crampy right upper quadrant (RUQ) abdominal pain, often related to gallstones. The pain can worsen with movement or deep breathing and may be relieved by changing positions. The patient's description of pain that waxes and wanes, worsens with sitting, and improves with sitting back, along with the absence of other symptoms like nausea or vomiting, makes biliary colic a plausible diagnosis.

Gastroesophageal Reflux Disease (GERD): Although the patient denies nausea and vomiting, GERD can cause epigastric or RUQ discomfort that may worsen with certain positions or activities. The crampy nature of the pain and its variability could fit a diagnosis of GERD, especially if the patient experiences heartburn or regurgitation that wasn't mentioned.
Peptic Ulcer Disease: Peptic ulcers can cause RUQ or epigastric pain that may be constant or intermittent. The pain can be exacerbated by food or improved by antacids, but the patient's denial of dietary concerns doesn't rule out this diagnosis entirely, as some ulcers may not follow the typical pattern of relief with food or antacids.
Hepatitis: Viral hepatitis can cause RUQ tenderness due to liver inflammation. The absence of jaundice, dark urine, or other systemic symptoms doesn't exclude this diagnosis, as early stages of hepatitis can be asymptomatic or present with nonspecific symptoms.

Cholecystitis: Inflammation of the gallbladder, often due to a gallstone blocking the cystic duct. It can present with constant, severe RUQ pain, fever, and leukocytosis. Although the patient's pain is described as crampy and variable, cholecystitis is a critical diagnosis to consider due to its potential for serious complications if not promptly treated.
Appendicitis: Although appendicitis typically presents with pain that migrates to the right lower quadrant, atypical presentations can occur, especially in young women. The absence of other symptoms like fever, nausea, or vomiting doesn't rule out appendicitis, and its potential for severe consequences if missed warrants consideration.
Pulmonary Embolism: In rare cases, pulmonary embolism can present with RUQ pain if the embolism involves the lung's inferior portions. The patient's description of pain worsening with sitting could be interpreted as worsening with deep breathing or movement, which might be seen in pulmonary embolism. This diagnosis is critical to consider due to its high mortality rate if untreated.

Hepatic Abscess: A rare condition characterized by a collection of pus in the liver, often due to bacterial infection. It can present with RUQ pain, fever, and systemic symptoms, although the patient's presentation lacks typical signs of infection.
Liver Tumors: Primary or metastatic liver tumors can cause RUQ pain due to liver enlargement or capsular stretching. These conditions are less likely in a young patient without a known history of cancer but should be considered in the differential diagnosis due to their potential impact on prognosis and treatment.