Do invisble retainers really result in an open bite?

I just got my retainers yesterday but I noticed that they are extremely uncomfortable. Why do these retainers make me have an open bite? I can only close my jaw by forcing the upper and lower front teeth to touch so when my mouth is at rest, it has an open bite and my jaw is starting to hurt from that. I asked my orthodontist and she said that it's normal but I just want to make sure.

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