When I started teaching people to walk I was very confident that I was onto something though there was one detail that surprised me at first.
A number of people returned from their initial sessions complaining of new pain in their lower back muscles that was different from the pain they arrived with.
Without much choice I asked people to carry on the way they were going and for everyone willing the new pain dissipated along with the old.
   
Everyone leans backward through life. As a result, most people have extremely tight lower back muscles such as multifidus and quadratus lumborum.
When I ask people to walk and stand differently there has to be an adjustment period as all sorts of muscles find a new home.
The intensity of the adjustment depends on whether you are naturally tight or not.
When I am teaching a yoga class and people say that something hurts in their lower back I always ask if the pain is in the center (the spine), or the muscles on either side of the spine.
If the answer is the muscles I am not worried too much. While it is possible that someone pulled a muscle, it is more likely, if I am teaching successfully, that they are using muscles differently and hopefully correctly.
Chronically poor movement patterns can make for some cranky and unhappy lower back muscles.
The process of aligning them correctly and training them to function properly can sometimes be intense but the payoff is usually worth the effort.