What stroke are you swimming? I will assume free style or front crawl if that is what you want to call it.
I was a competitive swimmer for 16 years and have been asked by many friends to help them with their swimming. As I watch them swim I see that pretty much all of them make the same major mistake along with a host of other problems. So I will tell you what that big mistake is and you can see if it fits with what you are experiencing.
Weaker swimmers tend to have trouble getting enough air during the breathing phase of their stroke so they lift their head rather than just turning it to the side. When you lift your head this causes downward pressure on the rest of your body, causing your feet and butt to sink. This results in you swimming with your body in a more vertical than horizontal position, less than ideal. You can experiment with this by simply trying to float, you will see that as soon as you start to lift your head your feet will head for the bottom and tipping your head forward will raise your feet. Personally I can’t just float so your mileage may vary on this.
Ideally your head position will be neutral, approximately what it would be if you were sitting down looking straight ahead. Try to pick a spot maybe a meter or two ahead of you on the bottom of the pool and look at that, when you take your breath just turn your head to the side and when you turn back to the center try to look at that spot again. Try to keep your head relatively level when you breath, if you do not have enough time to get a full breath in you are better off rotating your whole body further to the side or pausing your stroke than you are lifting your head. If you lose the horizontal position try to relax and stretch your stroke out to get yourself flat again.
If you are swimming another stroke the same thing applies, you want to try to be horizontal. If you watch strong competitive swimmers you may see exceptions to this when they are sprinting but when you watch distance swimmers you will see that they are flat in the water and relaxed.