You determined that she had committed an "illegal act" because she had been convicted. In other words, her conviction made the act illegal. And she would not have been convicted had the act not been illegal. The act of conviction determines the illegality and the illegality derives the conviction.
One more time ...
Post hoc ergo propter hoc.
You determined that she had committed an "illegal act" because she had been convicted. Convicted of that specific illegal act.
Post hoc ergo propter hoc.
The two events were not unrelated. Her conviction was in consequence of her illegal act. Post hoc ergo propter hoc refers to a coincidental correlation. This was causal.
Just because you can copy and paste a Latin term doesn't mean you comprehend its meaning.