It's very difficult to say anything meaningful based on so little information. It's sounds like there could be nerve damage and that could be caused by trauma, infection, or other causes. If just the lower lid appears droopy, sometimes it will look that way if the third eyelid (which covers the eye from the bottom up) is raised for some reason. This may occur because the eye is painful for someone reason or because the nerve supply to the third eyelid isn't functioning properly. Your best bet is to have you dog examined by your veterinarian so he/she can determine what's going on.
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