Am I experiencing Hering's Law after ptosis surgery?

If you only had one eye operated on, and the other one is now falling, that is an example of Herring's Law.  If this stays this way, which it may if it has already been several weeks, you may want to now lift the other eyelid.   

A photograph would be very helpful to discern the type of issue you are having.  Generally ptosis is bilateral with one side more ptotic than the other.  With an appropriately detailed consultation, it is generally straight forward to predict when both sides should be corrected at the same time and also to what degree.  It is also possible to get this wrong.  In the business we like to say if you are told about it in advance, it is an explanation.  If you are told about it after surgery, it is an excuse.  I suspect that a second eyelid ptosis surgery will be in your future.  Consider reposting with a photo for more accurate advice.

Yes, this is a possibility from correction of the affected eye. You may have to have ptosis repair on the opposite eye, but I would give it some more time to see how things respond with the swelling subsiding.