I had the same problem! I am a runner and in good shape, routinely running 8-10 miles at a time through the first several weeks of pregnancy, but during my first trimester I was out of breath just walking! My OB said it was just hormones and increased blood volume from pregnancy. But don't worry, it should improve! I'm now 20 weeks and the breathing is much better, so I'm taking advantage of being able to run and swim again while I can, before my lungs get compressed by the baby! 

I was out of breath in the first trimester too...just take it slow and give your body a chance to adjust! 

My very well controlled allergy induced asthma (didn't even have an active RX for inhaler since I hadn't had an episode in a couple years) flared in early pregnancy. Walking around the path at the park in my neighborhood set me off so it didn't take much. My OB said it is normal and wrote me a new RX for a pregnancy safe inhaler. I don't have to use it much but I do carry it. Talk to your OB, there are easy fixes.Good luck!

I'm out of breath all the time even now into my third trimester. But I also have asthma so that might be playing a bigger role than usual....