Anxious in Florida,
Welcome to our Fraternity that no man will rush to join. This is the right place to find the help and answers to your questions from our collective PX experiences, and maybe you will not be so "anxious in Florida".
I am neither an expert nor a doctor but with you lower PSA IMO I doubt you have bone mets from PC that is causing your lower back pain. Are you taking any ED oral drugs like Cialis or Levitra? If you are they could be the cause of the lower back pain. My URO had me on daily Cialis and I developed back and leg pains that stopped when I quit Cialis and went back to Viagra. A lot of other things can cause the pain you describe but I would not be concerned that PC is the cause.
Please stay in contact here, unless your biopsy comes back negative, and let us know what is going on, we are all here to help.
The Water Guy.