Is she not feeling well? Did you both have a fight? Maybe she is upset and needs time alone. If you woke up and found her there, maybe you were snoring or moving around too much so she went to the couch. Maybe she wasn't tired and didn't want to wake you up so she went to the couch but fell asleep. Only way to know for sure would be to ask her. But if she is sleeping now, the best thing to do is make sure she has a blanket and ask her in the morning.
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