I have a band-like pain under my breast line that increases with inspiration, what could this be?

Possibly muscular: Your pain is possibly due to muscle strain/spasm. But, there are also other potential causes. If you've done recent heavy lifting/strenuous exercise, had a cold wherein you coughed a lot, etc, then this is very much possible. But, considering your medical history, if you are having any chest pain, shortness of breath, or fever, however, you should see a doctor ASAP - go to ER. Best wishes.