Way to go on signing up w/ a club - I wish we still had one going here, I improved so much faster.

I used to get cramps when I did a heavy swim session. I looked up cramping causes and found 101 different reasons
I discovered I always got them when my muscles were fatigued. Once I started strength training more they let up (then I got a wicked one the 2nd lap out when doing a biathlon I have NO idea what caused that one)

Hope you don't get another one and if you do that you get to the root of the problem.