I know what you mean. The same thing happened to me; as I understand it, for me, before I was out I NEVER would have expected to be seen to the outside world for who I was and was resigned to my fate. Now, I realise that I am the master of my own life and I want to be seen as ME. I never made an effort to look like a 'girl' but I didn't make an effort to look how I wanted to - like a dude; now that I do, it feels like an insult to my 'attempt' if people get it wrong.
So it's like, pre transition, why would you be angry at people calling you 'she' when you've made no effort to show them otherwise or there is no reason for them not to call you she... But then when making the step to appear more masculine, people then get it wrong, that's very frustrating.