I'm curious about the OP's symptoms. Does the burning sensation continue if you stand and continue pedaling out of the saddle? Or does it lessen? I'm thinking that you have something going on with your saddle and pelvis, with some pressure impinging on nerves.
Have you tried rolling your pelvis forward or backward gently while pedaling? Does the problem lessen over time when you ride (not as bad 30 minutes into the ride as it had been only 2 minutes in)?