She was obviously a little Butthurt about you wanting to only go on shopping trips with 'Her' but are not comfortable with her including her daughter & granddaughter,
and just as obviously overtly hit back with some sarcasm.
I'm a "Guy" so don't ask me to understand about "what it is with 'Women & Shopping" ,,
If there's anything I might understand,
it's that Women with their Daughters & Granddaughters are often a 'package deal' , to-wit One sometimes doesn't come without the Other in all things including but not limited to even sometimes shopping with friends.
it appears that your friend may have felt that you might have considered her Daughter & Granddaughter to be an "Imposition" on the intimate camaraderie you enjoy with each'other when shopping together while proposing a nice "Girls day out" that would've included her closest family & a closest friend,,hence the backhanded remark about "Forcing' or imposing",,
LOL!,,for all I know, in another culture an invitation like that might be some kind of 'Honer' and declining considered something of an insult.
But far be-it from Me to make any assumptions about the common Culture of Women or intimacy issues in any of their Shopping-rituals!,,hahaha!
Something similar might be like a couple of "Guys" that are accustomed to only going bowling or Fishing with each'other then all of a sudden one wants to bring a visiting Brother in-law along,
and since not everybody does the same things the same way, in ways that can sometimes be inconvenient or even a little annoying, things can tend to feel a little awkward.
With 'Guys' it could be something as inanely dumb as the 'other' being introverted, extroverted, flamingly-gay, smokes, drinks, cusses, has a weak bladder or just isn't as familiar with the common activity.
Either way, Shopping is just Shopping and Fishing is just Fishing and for Men & Women one is no-less enjoyable than the other depending on perspective.
I don't know what to tell you about how to heal the minor accidental scratch with your friend,
but you might consider inviting them all to do some kind of special Girly-thing that you're comfortable doing with a few more girls along...