Viral Exanthem: Given the presentation of a low-grade fever followed by a full-body rash and joint symptoms, with the rash being non-itchy, non-painful, and not raised, a viral exanthem is a plausible diagnosis. The fact that the rash resolved but joint pain persists could indicate a post-viral arthritis syndrome.

Lupus: Although the initial screening tests (e.g., RF, CRP, ESR) were normal, Systemic Lupus Erythematosus (SLE) can present with a wide range of symptoms including rash, joint pain, and fever. The absence of specific antibodies or abnormal lab values does not rule out lupus entirely, as it can be seronegative.
Rheumatoid Arthritis (RA): The joint pain and swelling could suggest an early presentation of RA, especially if the symptoms persist. However, the normal inflammatory markers and the absence of morning stiffness or other typical RA symptoms make this less likely.
Parvovirus B19 Infection: This virus can cause a rash and joint symptoms, particularly in adults. The fact that the rash was not itchy or painful and has resolved could fit with this diagnosis, although joint pain can persist.

Lyme Disease: Despite the tick panel being normal, false negatives can occur, especially if the test is done too early in the course of the disease. Lyme disease can cause a rash (erythema migrans), fever, and joint pain, making it a critical diagnosis not to miss due to its potential for serious complications if untreated.
Endocarditis: Although less likely given the presentation, infective endocarditis can cause fever, joint pain, and sometimes a rash (e.g., Janeway lesions). This diagnosis is critical not to miss due to its high mortality rate if not treated promptly.
HIV Serconversion Illness: The symptoms of fever, rash, and joint pain can occur during the acute seroconversion phase of HIV infection. Given the potential long-term implications, missing this diagnosis could have significant consequences.

Adult-Onset Still's Disease (AOSD): Characterized by high fevers, salmon-colored rash, and joint pain, AOSD is a rare condition that could fit some of the patient's symptoms. However, the lack of a high fever and the specific characteristics of the rash make this less likely.
Sarcoidosis: This condition can cause joint pain, fever, and sometimes a rash. Although rare, it's a diagnosis to consider in patients with persistent symptoms and no clear alternative diagnosis.
Relapsing Polychondritis: A rare autoimmune disorder that can cause joint pain, fever, and sometimes a rash, among other symptoms. The diagnosis is challenging and typically considered when other diagnoses have been ruled out.