Dear Dylan,
Have you noticed that all the arm chair experts that have answered your post so far have not even asked you your age, weight, previous medical history, medications you're taking, anything about the nature of your breathlessness or any associated symptoms nor do they have the opportunity to exam you or do any investigations. I would treat their advice with caution. See a proper doctor. Even if it turns out to be nothing it will put your (and my) mind at ease.
Regards,
Mark57