Forgive yourself hun. I doubt there are many mummies who haven't done something similar (me ). You know it's not good, but there really are strong mitigating circumstances here. It was the same for me - I handled it ok for the first 12 months or so... then... oh dear. I've lost it a few times. The last time I yelled at DS was just a week or so ago (he had a cold too! oh that makes it worse I know) and after he'd calmed down he snuggled up to me and said "mum told me off".
But we have to let these things go and move on. He won't be permanently scarred by ocasional slip-ups, I'm sure of that.