There can be many reasons for this, but it's best to ask, see your doctor. Opioids can suppress the respiratory system, it does with me, making me out of breath, and can causes difficulty swallowing, tongue swelling, but with treatment that is a cause also. Then pneumonia can cause difficulty breathing, which I had several times, some were from aspirating. Dehydration can be another. If it's that bad, you can always go to the ER.

Good luck with the rest of your treatments, and tell your medical team.