Do most of my swimming in the pool. Rarely have a chance to get to open water for any long swims before races. During 1/2 and full races, usually in my wetsuit, my lower back will start to tighten up maybe a mile into the swim. Then it translates into a tight lower back during the bike.
On my long training bike rides, the lower back doesn't bother me at all so I'm pretty sure it's coming from the swimming.
Was on vacation last week and was able to get in a 2 mile ocean swim, no wetsuit. Around 1 mile the back started to tighten up again.
Not sure if it's from looking up to sight so I'm arching the back and it's not used to that, whereas in the pool I'm looking at the bottom of the pool the whole time plus with flip turns the back it getting a little stretch each time.

Anything to do to correct this. Have been doing some lower back strengthening exercises this past winter but not sure it doing anything.