I agree with Wannabe.
At 11 I would have been upset by that comment - I would been thinking - I have a mum, I don't want or need another. You're my dad's girlfriend, now you're talking about marrying him?! Etc, etc, etc.
Has your dp ever mentioned the possibility of you getting married to his dd or was this the first mention? If so that would also potentially have upset dsd massively - no matter how long you have been a together she may not have joined those particular dots.
You were talking to an 11 year old - not an adult. To you it may have just been a throwaway comment about something you have already considered and processed - but an 11 year old you are Dad's gf and she clearly HADN'T made that connection yet, so your comment was actually a bombshell in HER 11 year old world.
She's gone home upset about it, when you are a child there are often a lot of churning emotions around the thought of parents remarrying, and her mum has had to deal with the fallout of what you saw as a throwaway comment, but that actually had a far larger significance for your dsd, so she is pissed off.
It's NOT just a name to a child, it holds vast amounts of meaning.
I say this as someone who has a stepmum. Who she and db and I have always referred to as 'dad's wife' because that's what she is, we get on really well and always have - but she isn't a 'mum' figure.