OMG!! I feel for ya! I have the same kinds of issues at the start of my period, and I'm not sure I would have pushed through it like you. Be proud of yourself If you enjoyed it at all - a big IF - I'd try it again, especially since this was just your second time swimming in open water.
I have my first tri on Tuesday evening - I'm starting small, a SUPER MINI Tri, so I can get a feel for open water with lots of people around me, transitions, and the whole experience before I do a sprint tri.
I will be thinking of you, and if you did what ya did last night with how you were feeling, surely I can get through this little tri