Could the situation conceivably be against a law? Yeah, probably. Contributing to the delinquency of a minor comes to mind (but not soliciting, given that you weren't seeking to meet). There are situations where it doesn't matter if the underage individual lies about being underage and the adult is still considered responsible. So, yeah, there's risk.
However, I seriously doubt that any law enforcement agency, as overworked as they normally are, would consider this situation worth their while to do much about. You were not trying to discover any of her personal information, you were not trying to meet in person, and you were not pursuing her because she's underage. With how immediately you dropped contact, you also aren't going to raise any red flags by continuing the pursuit now that you know. There's just not much there to be worth the time and expense of law enforcement to pursue.
Sure, the mother was upset. I'd have been, too, what with all the horror stories that are all over the place about what happens to girls who 'talk dirty' on the internet. Her severe tone probably had a lot of panic in it. You can't know what else that woman had just seen as she reviewed what her daughter had been doing. Just because you didn't seek personal information or talk about meeting in person doesn't mean that was universally the case. I can totally appreciate how you feel about the threat, but it's doing what the woman wanted, which is cutting off contact with people she believes put her daughter at risk of kidnapping, rape and death. You know you weren't a predator, but that woman doesn't, and there might have been predators in other material she read before telling everyone on her daughter's 'flirt list' to back off.
Everything honestly should be okay, and the sooner you get back on the horse, so to speak, and do a bit of rp, the less likely this will sour you on it long-term. I'd offer to write with you, but I'm still getting myself back up to speed after my summer's complications.