Feeling exhausted

I started regularly swimming in a pool a few weeks ago. This is the basic change in my exercise plan, and I aim for 3 times per week, 30 minutes each time. I started with 16 25m pool lenghts the firest couple of times, I have managed between 24 and 28 the last week, at the same time, so not super fast or long. The thing is, while I love the workout, I feel completely exhausted the next day. I am not out of shape, have been consistently exercising for years. Swimming is mainly replacing jogging in my routine, so my aerobic capacity is pretty good too. I am just wondering why I am feeling so tired. I am not trying to lose weight and have not been restricting calories or changing anything in my usual diet, so that's not it either.
It is not that I am sore, it is that I feel just tired, like e.g. I would normally feel the day after going for a several hours hike. Am I right to assume that adapting to swimming is causing this, or should I investigate further with a dr? Had extensive blood work done a month ago, and everythign was fine.