Presumably back spasms or lower back strain. Very, very sharp back pain immediately following the District 2 3200m race. I'd been noticing back stiffness sitting in the uncomfortable bleachers during league and district championship weeks this week and last. However, it seems like it was aggravated during the race and when the adrenaline went away after the race it became hard to walk normally. Stayed in bed the day after.
Stretching, PT, lots of rest. Pushed back start of XC summer training to recover. Occasionally very minor dull lower back pain on the R side still bugs me so it's never been perfect, but is pretty much gone.