I would have thought if it was a collision, the bang would have woken you before the rocking.

Thought it could be caused by an anchor stock, but it sounds to low down to be that.
Can't imagine it would be a dinghy (not heavy enough) or a jet ski (in the dark??)

Bit of a mystery I'm afraid.

Sorry to hear of your plight, and hope it wasn't too painful to fix.

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