I am sorry that you are having problems.
Twitching of the eyelid is a common symptom which is nearly always harmless. It will usually resolve after a few days or weeks.
On the other hand, relatively sudden drooping of the eyelid, a condition that we call
ptosis, especially in a young person, is much more suggestive of a neurological condition. There are two systems of nerves which keep the eyelid open and if there is an interruption of either nerve pathway, the eyelid will droop.
The eyelid can also droop on the basis of conditions which affect the muscles which keep the eyelid in a normal an elevated position. An example of such a condition is myasthenia gravis.
The concern for a neurological condition is increased if you have a new onset of headache which started at the same time as a drooping eyelid. A normal MRI is good news but this does not look at all of the nerve pathways and the MRI can't "see" very small abnormalities of the blood vessels which surround the nerves which keep the eyelid normally elevated.
With all of this said, the best doctor to evaluate this situation would be a neuro-ophthalmologist. This is a doctor trained in both neurology and ophthalmology. You can find one by going to www.AAO.org and selecting "Find an Eye MD" from the top tabs and then searching for a specialist in "Neuro-Ophthalmology". If one is not available in your area, the second choice would be a neurologist.
Because of all of the possibilities including some that require immediate diagnosis and treatment, I would make the appointment today.