I have been having painful mid back muscle spasms.i think?it just came on out of the blue two days ago.i'm not really active so what is this?

Back pain.: It could still be spasms or a back strain. You don't really have to be active to get one - actually, inactivity could put you at risk for injuring your back simply because you haven't been building up muscle strength. If some over the counter meds like tylenol (acetaminophen) or Ibuprofen don't help, a checkout with your doctor is reasonable - especially if you're having any numbness or weakness in your legs.

Non specific symptom: Just to illustate the range of causes abnormal posture can be a cause--scolisis, back sprain, internal organs like kidney and pancreas can cause lbp and cause you to spasm your muscle as a protective reflex - simple radiculitis from diabetes, shingles -then of course problems with disc--if persist need exam by doc.