Hy Laura
No it's nothing new feeling tired/exhausted after exercise of any type.
You should have been advised of what to do when you increase you activity levels above your normal routine.
I know it's a lot to take in at first but the increased activity one is important.
I use to play a lot of badmindton and for that I took a bottle of sugar drink [lucozade or similar] and sipped it after each game, I did play in league matches so they were quite competitive and therefore high in exercise.
Your normal insulin dose is for normal activity but for INCREASED ACTIVITY you need to up you energy source by eat/drinking some extra carbs. I find the sugar drink the easiest and quickest way to get quick carbs into my system when I'm exercising.
Swimming. it is important you take some extra before you start your swim as you don't want to go hypo while in the water.
As always good blood testing is the key so before exercise do your check and in the initial first times also while you are doing the exercise that way you will build up a memory map of how much you will need for different exercise regimes.
Don't forget you can also ask your diabetic nurse with any questions.
Regards