In answer to your title question, I haven't personally seen a female slug out if cycled and not bred, though it certainly would be possible. Though it's clear that ovulation is induced in pythons, exactly what induces it hasn't been clearly studied.

It's also possible that she was bred to the male a long time ago and retained the sperm, but that by the time she ovulated, the sperm was no longer viable.

I once had an imported blood python drop eggs for me nearly two years after I acquired her. She was never once in with a male while in my care ... Because she was sold to me as (and I never doubted that she was) a male.

I also might recommend switching her onto small rats or at least trying to get big, but not fat, mice into her to get her weight up. I've seen overly-fat mice give snakes diarrhea, which would probably be counterproductive if you're trying to put weight back on her.