Ah Mama, you were honest about what your could (or in this case, could NOT) do and that's ok.
I TOTALLY get why you wouldn't want to and I've always admired Moms who were all too willing to take in youngsters and sometimes wished I was one of them but, well, I'm not.
When my friend and I had onlies, we used to take turns once per week trading the babysitting. It gave us each a morning off. Then, when she had her second she didn't feel capable of babysitting for a time which I totally understood. She offered to pay me for that one day a week with her toddler and I accepted. One day a week, I could do that, and the occasional emergency drop off was ok too. I too envisioned raising MY child and didn't feel comfortable with an arrangement that included more than 4 or 5 hours, once per week. Funny, I've had people tell me I'd be a great daycare provider and have asked if I had an interest. I had to decline. For all my "talent" for it, and love of children, I just didn't desire it, yk? We could have really used the money, but I strongly believe a daycare provider (no matter how many or how little children) needs to really want to be doing it. I liked it a lot... one day a week. Anymore and I know it wouldn't have been a good fit.
It sounds like you offered up services to her in an emergency and that has to be of comfort to her. I agree for her to assume that you were willing to take on her DS full time was a BIG stretch, but of course do understand why she'd have loved that for her DS. Your home sounds a loving and caring environment, mama.