Could someone please tell me...

...when did saying 'excuse me' become rude?
Compared to the other variants of 'get out of my way' and 'shift it', this phrase seems polite to me, and I was always under the impression that this view was shared by the majority of society's population.
However, somewhere along the line, for the person I said it to today, it became offensive. Upon casting my mind back, I can safely say that no intonation or body language could cause confusion as to the context I used it in, I merely wanted to get past a pair of dawdlers.
There I was, having pur-chased some shopping from the supermarket, and making the small treck home. I am quite a fast walker so it is a natural occurence that I walk past people who are walking at a slower speed than me. Well today, I encountered a lady and what I presume (and hope) was her grandson. Little toddler was clutching his granny's hand and standing as far away from her as possible but so that he was still able to maintain this physical contact. This queer position caused the path to be blocked, and I was unable to get past. I stood behind for a good while, waiting for a loophole in this cunning display, but none came.
So, I did what any person would naturally do in this situation. The lady (and I use this definition in it's broadest terms) didn't know I was there, so I alerted her attention, with a polite 'excuse me'
As she stepped to the side, I thanked her, smiled and walked past.
However, as I did so, I heard an audible tut and an exclamation of: 'well there's no need to be so ignorant'
I could understand this if my tone was frustrated, or I offered no acknowledgement of thanks or a smile, but this setup was as polite as could be and still, wanting to get home before dusk fell classes me as ignorant.
Now, I'm sorry, but with hooligans, chavs and thugs littering the streets more than ever, I would be grateful to receive an excuse me, a word of thanks and a smile.
All I normally receive is a shove.
So i'm sorry if I'm ignorant, but I could have been a hell of a lot worse!