What Causes Difficulty Breathing While Doing Cardio?

I am an active 20 year old college student and I do strength-training exercise and cardio at least 2-4 times per week. Lately I feel like I have a hard time catching my breath during cardio, and when I am at rest sometimes I feel like my lungs aren t taking in as much air and when I try it almost feels like my diaphragm isn t moving down enough for my lungs to expand, if that makes sense. Should I be worried?