I'm so sorry that you have struck this bad run first up! It is probably just plain bad luck that you have picked up an infection at just the wrong moment. This happened to me on my 4th cycle and I have never worked out where it came from as I was always pretty careful to avoid crowds etc and like you had good general health and fitness.
I knew I was pretty sick when I actually wanted (not quite the right word) to go to hospital. I had 3 days in and then felt pretty ordinary for the rest of that cycle. It really shook my confidence and it was pretty hard to get my positiveness back. I had the Neulasta injections for the rest of my chemos and never had another problem but it does take a lot out of you and I never felt as good after that. Hopefully you will bounce back as it is only your first round so your body will be in a better position than mine was by round 4.
You are right when you say that cancer journeys do not always go to plan but you have the sort of attitude that will have you readjusting soon. Give yourself a chance to get over this bump and then get back to the next step. Hope you are feeling better soon. Sending lots of good wishes for a better time next chemo. Take care. Deanne xxx