Not necessarily.  Is the pain worse if you sit for a period of time and then get up?  Are you stiff in the lower back in the morning?  Also anklyosising spondylitis does not involve muscles at all.  It arthritis starting in the lower spine and they bodies way of coping with it is to make more bone on the spine thus fusing the spine.  As the fusing goes up the spine it can spread into the ribs making it difficult to breath.  I would go to your doctor with your concerns because this is not something you leave.  If you have it you must remain active (yoga is great)  you need to fix your posture, if fusing occurs you spine be set in the position you left it in.  You don't want to be hunched over.  But the good news is that the biological drugs are there to prevent this type of arthritis from getting out of control.  I have it and have nothing but pain in the  lower back.  I take enbrel twice a week and arava.  I work out on a daily basis.  But again you need to go to your dr.