How long has it been a problem on this occasion?
Any pain on passing urine, strong odor or change to color of urine?
Any problems with nausea, vomiting?
Is the pain constant or intermittent? On a scale of 1-10, (ten is worst pain) how bad?
Does it prevent you from sleeping?
This does not typically sound like a urinary tract infection which would normally be shorter in duration with either urinary symptoms or sustained fever. If this was a kidney stone then we would expect the pain to move into the groin or flanks after some time. Also if it was a kidney problem then we would expect the pain to be present regardless of position.
So a pain that is related to position is most likely a muscle or skeletal problem. This warrants a full history and examination, but you may also need a MRI spine in order to fully understand what is happening.
Do make sure that you sit in a good position with a well supported chair. I would make an appointment with your family doctor or an orthopedic doctor for initial assessment. If the fever were to occur again then certainly have the kidney checked too, but at the moment I would pursue the musculo-skeletal cause first.