I agree with The FallenMadonna. This discussion has gone off on a few tangents, but going back to your OP:
I think it was inconsiderate to change your DS on a bench when someone was sitting next to you. Newsflash: Not everyone thinks your loved one's bum is adorable (I don't mean this harshly, as I adore my own LOs chubby bottoms, but I know not everyone else does)
The woman clearly felt there was a problem, hence the tutting.
In response, instead of considering the impact your actions were having on her, or trying to smooth the situation, you said 'have you got a problem?' I think this is a pretty aggressive thing to say to a stranger.
You ask on MN 'So was i wrong? Or was she wrong?'
The vast majority of responses have suggested that it was not an appropriate thing to do given the circumstances (or at the very least they would not have done it), yet you have continued to defend your actions to the hilt.
In response to your original question, yes, I think you were wrong, inconsiderate, and aggressive towards the poor woman. Otherwise I am sure you are very nice