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What you describe sounds like hypnogogic hallucinations(+/- sleep paralysis) which do not indicate a mental illness.

If you are not having such experiences at any other time of the day, or indeed any other new odd symptoms then I doubt you have too much to worry about.Try not to torment yourself looking for esoteric explanations for what is a fairly common occurrence in people who are in no way ill.

These hallucinations can be seen in narcolepsy, in psychosis (but almost invariably with other features) an on occasion with certain types of seizure disorder.