lumbar pain in a race for the first time

So, yesterday ran Belews Lake Int'l for my first tri in 17 years.  Was, for the most part, happy with my result, although I got dinged 2 minutes for forgetting my race belt heading out of T2 and thus running without a bib number.  Was sub 3, transitions felt efficient - I need some work on open water swimming (I was a collegiate swimmer, and didn't train the swim as hard as I did the other two legs... kinda underestimated).

at ANY rate... the reason for this thread is that for the first time, I had some pretty severe lower back tightness and pain at the start of the run, which went away about 1.5 mi in - but resulted in a very slow run/walk start over the first part of the leg.  I have done brick training for the past several months with no real trouble, and never have trouble in stand-alone runs.  My back wasn't really hurting on the bike at all, and I was just fit on the bike in November which has helped my power numbers a good deal.  NB - I don't own a tri bike.  I rode my road race bike in the drops - it's set up in a pretty aggressive position.  

Would love to hear thoughts on causes and thoughts on things I can do to address it both in the longterm and during a race (other than HTFU for the latter).