I had a friend at that age who was trying to get pregnant but she picked guys who she didn't know to have sex with, because she didn't want them involved. Her mother (who was married to her father) had recently had a baby and I wondered if that was somehow related but I don't know for sure. She seemed pretty well obsessed with it, though.
She had her baby at fifteen or sixteen, lived with her parents for a while, then moved out and eventually got married (not to the baby's father, who I don't believe was ever told or even in touch with her again after a one-night stand).
I am sure she's not the only one because I knew a lot of girls who had babies as teenagers. And, as you say, it's likely to be kept quiet if it was deliberate. Even a teenager would know how poorly most people would think of it, is my guess. I was surprised this girl told everyone about her plans.
I also know a few girls who got married at that age when they weren't even pregnant but just wanted to. I don't think it necessarily is from a bad home life, though that could certainly be one reason.
Anyway, I don't think it means you were mentally ill. If you were, there would have been other, more definite signs of it. Sounds to me like you were just being a fifteen-year-old. That's an age where kids often want to be grown more than anything, but tend to have such little life experience that they still think they know it all!
Glad your life has worked out well. :)