I don't have any kids, but I have noticed that "gross" can mean a lot of different things. Vegetables are gross, just like bugs are gross, boys are gross, and your parents kissing is gross. I have stretch marks on my arms, too, and I am very self conscious about them. I know they're not gross, though... just different.
Like you said, I think the pain comes from her pointing out something that you're so insecure about. One of the hardest things to remember, for me at least, is that I'm NOT fat anymore. I don't think that anyone would ever assume that I have so many, because stretch marks come from skin stretching and at this weight I don't look like I've done a whole lot of stretching. So, a random person sees the stretch marks on my arms... and I really doubt that they'll be thinking, "Ewww!!" or "OMG she used to be FAT?! FOR SHAMEEE!!"
In all seriousness, I would think that they would be more interested in how they got there, KWIM? But really, I don't think people even pay enough attention to notice.
Just know that you're not alone.
