What could cause constant pain in the inguinal region and tenderness around the lymph nodes, along with a burning sensation midway down the right leg?

Several things: Burning is usually a sign of local inflammation of tissues or pain resulting from nerve injury/inflammation.Causes of all your symptoms could include infection (could be serious), inflammation of peripheral nerves (ex. femoral, genitofemoral, obturator, saphenous), blood vessel inflammation/infection, etc. Serious enough, esp. w/fever, to warrant visit to dr. ASAP.