She must be a mother.
I'm sure women could only producce milk when they have had a baby
No, but that is an assumption that a lot of people (mainly guys) make. Just like some people think that girls with bigger breasts produce more milk than girls with smaller breasts. That also has little or nothing to do with it. There was a girl on the American version of Big Brother several years ago who made a spectacle of herself by running around squirting milk from her tits (I don't know which season as I've never really watched that show). AFAIK, she'd never been pregnant, but said she'd been lactating since she was in her early teens.
You can even induce lactation in a woman by using those "nipplette" suction devices periodically over time. Apparently they stimulate production of hormones that cause lactation. Diet also influences it. But yeah, girls can and do lactate without ever being pregnant. I've been with two girls who were lactating and hadn't been pregnant. The first was kind of embarrassed by it, but the other one was freaky like that and she made the party. I was kind of young and it took me aback at first. But then the freak kicked in on me and I got into the scene too! :nanner:
I'm not into the squirting fad
(some girls can actually squirt during orgasm, but more often than not, many are just peeing), but milky tits can be pretty fascinating.