Dealing With Setbacks

So, my first few days back in the gym were comical, but my third was downright scary.  I was doing some squats with the bar and the first few sets went really well.  My third, when I lifted the bar and the weights up, I felt intense pain in my lower back.  This was Thursday.  Today, Saturday, I am still gimping around my house like an 80 year old.  Usually I would just be annoyed by an injury of this nature... however I am scheduled to do another 50K next Sunday.  Not entirely sure at this point if that'll be able to happen.  Right now, I'd settle for being able to stand up straight and walk around my house.