My aching back

I've been suffering from lower back pain for several weeks, most likely due to sitting on my bed for hours at a time checking exams last month.  I won't be stupid enough to sit that way again, but I can't seem to get rid of the pain in my back.  Today, it was so bad that I had to stop my swim workout in the middle.  I was barely moving in the pool because I couldn't properly rotate my hips.  Ouch.  Strangely, my back only bothers me while swimming -- I don't feel a thing while running (not that I've been doing much of that) or while cycling.  That's a good thing, because tomorrow I have a 90 km ride planned.  Of course, if my back doesn't get me, the heat will -- it's supposed to be close to 100 degrees tomorrow (again).