hi Linda--
From what I have read, the cardiac toxicity risk from Herceptin isn't something you will feel as you have described--they are risks that slowly over time will weaken primarily the left ventricular function, which isn't something that you will feel for a day or two, then will go away. It's something that needs to be evaluated as you continue your care.
That being said, you definitely need to tell your oncologist, because he/she needs to know about your symptoms. I had some chest pains after a month or two of Herceptin/Taxol/Carboplatin, and it turned out I had a respiratory infection. Just yersterday I went in because I was very short of breath. They couldn't rule out a respiratory infection, so they put me on antibiotics just in case. But my symptoms could have been as a result of the radiation I just finished that was directed toward my chest wall, which could have caused some inflammation in the muscle surrounding my lungs--it had nothing to do with my Herceptin. You could be experiencing symptoms of something else, so please talk to your doctor.
My opinion on Herceptin is that it's life saver, and if I personally started exhibiting cardiac instability, I would make a decision by evaluating the risks of continuing vs. the risks of discontinuing Herceptin. At that point I guess I'd be choosing between the lesser of two evils :-)
For your further information, attached is the letter from Genentech to physicians about Herceptin and the cardiac risks, and below is the website section that deals with the warnings associated with Herceptin:
http://www.herceptin.com/herceptin/physician/pi.htm
Good luck!
Val