Hi Tany, thought I'd fill you in on my story while on Herceptin, this may not apply to you but perhaps worth checking out with your oncologist?
I finished Herceptin in July 2013 and although, like you, I felt OK when I was having it on its own after chemo, I did start getting very tired and weak and a bit breathless. I mentioned it to my oncologist and she arranged for an ECG & ultrasound to check my heart function. These tests showed that the Herceptin had caused some heart problems (cardiomyopathy). For the last 6 months of my treatment I also had to see a cardiologist regularly to be more closely monitored and I was on heart tablets and my Herceptin dosage was changed. Thankfully, the issue was temporary, and my heart function went back to normal within about 6 months of finishing treatment, so I no longer see the cardiologist and I don't take the heart tablets anymore. I was really worried when they first told me about the heart issues, but the close monitoring and other measures meant that I could complete the treatment, which was really important and it provided me with some reassurance.
As I say, this may not be the issue for you, and I hope you are able to find a solution and wish you all the best for the remainder of your treatment. Jane xx