"I was so shocked when people were shocked by it," Diandra tells the Telegraph. "It's a natural thing."
"People are more used to seeing naked bodies in more of a sexual way than a motherly way. I'm not against that at all, I've done nude pictures and I'm very proud of my body, but it's okay to show your body in that way. If you're pulling out your breasts to feed your child, that should be even more so okay," she tells the publication.
However, Forrest wasn't nursing Rain as she walked for designer Gypsy Sport – she had already fed her backstage.
"I was the last out, so other people were walking and I was nursing her because she was getting a little cranky," Forrest says. "Her mouth would have had to be on my boob to be breastfeeding! So I was like 'what?' "
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"It's hard to breastfeed in that position, so it would have been a little impossible on a runway," Forrest says. "Because she's so small – if she could hold her head up that would be easier, but you can't really walk and breastfeed."
Forrest, who walked in the same show for Gypsy Sport in September while she was pregnant with Rain, said the idea to walk with her newborn started as a joke.
"It was funny because a few weeks after she was born I knew fashion week was coming back and I lost my baby weight so was happy about that and me and my agent were joking like 'we should do the Gypsy Sport show again, breastfeeding or just walking with the baby down the runway.' We were just joking," Forrest explains. "The next week the designer called us and asked us if it's something we'd be interested in. We took it and ran with it."
But now she's frustrated with all of the criticism towards breastfeeding in public.
"You need to get real, it's really just a boob," Forrest says. "Men walk around topless, and women walk around topless, and if they can do that to be sexual, we can do this for our babies. Whether we're doing it modestly or not, are covering ourselves or not, which I do."
"It's something we should be proud of and women should do more if they can. People are ridiculous."