The two general reasons for involuntary hospitalization are if you are considered a risk to your own safety or the safety of others. Your therapist was basically responding along that line and letting you know that those kinds of thoughts, if in any way put to plan, would have to be taken seriously. Perhaps she was trying to impress on you the necessity to take your own thoughts seriously and not casually. We can get into habits of thinking that can become ruminations and sort of take over. Been there about suicide myself. Breaking that thought process can become extremely difficult.

It's hard to know without much detail about what was actually said.

That said, I doubt your therapist would hospitalize you over thoughts alone. Talk to her about how her reaction took you aback. Talk with her about the difference between thoughts and plans and actions. Get on the same page. That is a process of discussion that is really important in therapy if you are prone to those kinds of thoughts so that you can feel safe to bring them up when they are happening. I don't know how long you have seen this therapist, so it may be a matter of you getting to know each other a bit more.