Pulled out my back??

I think I may have pulled out my back.  I'm 38 weeks tomorrow and I know that this baby is bigger than my last born at 41 weeks.  I was on my feet for almost 5 hours yesterday and today I'm on excruciating pain.  The pain is in the upper left side of my back.  The only way to relieve pain is to put pressure on it by laying on a mountain of pillows.  Could it be anything else??  I've also had a racing heart beat and having trouble breathing..Google said it could be like a clot in my lung which also feels like a pulled muscle.  What do y'all think? Chances are it's just a pulled muscle eh?