J.W.
You need to stop explaining it and let her question. The problem when you explain is you don't know what her needs are so you add information that she didn't need and now confuses her.
You say she is intelligent then have faith in her ability to seek what she needs.
You are making connections that she may never make herself, the going to the hospital and not coming back. By doing that you are going to set her up for panic the next time you have to go to the hospital. By letter her question she will probably ask what is different, or just a simple, you are coming back right? Yup!
Adults don't think like kids, we need to always remember this, especially when we are trying to explain complex issues.