Hypothyroidism: This condition can cause low-grade fever, bradycardia, and potentially palpitations due to the heart's decreased demand for oxygen. The metabolic slowdown associated with hypothyroidism can also lead to a feeling of palpitations.

Anxiety or Panic Disorder: These conditions can cause palpitations and may be associated with a low-grade fever if the anxiety leads to increased muscle tension or other physiological responses. Bradycardia can occur in some cases, especially if the individual has a vagal response.
Viral Pericarditis: This condition can cause chest pain (which might be perceived as palpitations), low-grade fever, and bradycardia due to the inflammation of the pericardium.
Medication Side Effects: Certain medications, such as beta-blockers, can cause bradycardia and potentially palpitations as a side effect. Low-grade fever could be unrelated or due to another condition.

Endocarditis: Although less common, infective endocarditis can present with low-grade fever, palpitations (due to heart valve issues), and bradycardia in some cases. It's crucial to consider this diagnosis due to its high mortality rate if left untreated.
Myocarditis: Inflammation of the heart muscle can lead to palpitations, low-grade fever, and bradycardia. This condition requires prompt diagnosis and treatment to prevent serious complications.
Lyme Carditis: A known cause of heart block and bradycardia, Lyme carditis can also present with palpitations and low-grade fever, especially in endemic areas.

Cardiac Sarcoidosis: A rare condition where granulomas form in the heart, potentially leading to bradycardia, palpitations, and systemic symptoms like low-grade fever.
Hypersensitivity Pneumonitis: An allergic reaction to certain substances can cause inflammation in the lungs, leading to symptoms that might include low-grade fever, palpitations (due to hypoxia), and potentially bradycardia in severe cases.