Drop in MUGA

Good morning. I have been on Herceptin for 4 months. My MUGA has gone from 69 to 55. My oncologist wants to repeat the MUGA in 6 weeks. That is all. Since I have a drop of 20% should I be a bit more worried. I noted to her prior to this last MUGA (last week) that I have been coughing and have chest tightness. Shouldnt I stop herceptin every three weeks for now? Is this reversable? Any med's to help my heart.

Please offer any advice you can. I appreciate you all. Kim 43 years old. Minneapolis. (p.s. done with AD/T now chest wall radiation. On my 19th of 30)