Swimming after run causing cramps

This has happened to me several times at the gym.  I run a few miles on the treadmill then jump in the pool for a short swim.  Sometime towards the end of my swim, often during cool down, one or both of my ankles short circuits and locks up.  My foot pulls up and in.  It's not terribly painful and if I grab it and pull it down and out it's not even noticeable as long as I hold it until the muscle releases.  Unfortunately even when the cramp is gone my ankles are weak and sore which ends my workout.

After the first couple of times I started doing some extra calf and ankle stretches between the run and swim and that usually did the trick, although occasionally I had to stop mid-set to stretch again.

The last two times I've been in the pool it's happened again regardless of the stretching.  The only thing that's different is that I'm now doing flip turns although I always pushed off the wall even when doing open turns.  Any ideas on why this is happening and how to stop it?