I can't seem to breathe when it's too hot outside. Is this normal?

Possibly. Yes, sometimes it can be difficult to breath when it is very hot outside ...
No. Difficulty breathing can be from being overweight, lack of exercise, asthma, or other heart, lung or medical problems. Shortness of breath should be further evaluated by your doctor. If you have not exercised regularly in a while, are very overweight, over 50 or have any chronic medical problems, you should see a doctor before starting an exercise program.