Sudden droopy eyelid of only one eye after eye drops at doctor?

Side effect of drops. This is usually caused by the eyedrops used during you eye exam. These can sometime cause temporary weakness of the eyelid muscles. This should resolve with time. If it persists you should return to your doctor for evaluation.
Ptosis, eyedrops. Most common eye drops instilled are pupil dilates and can elevate a mildly drooping eye so if not in both the other eye might look more drooped. Conversely, steroids topical in the eye used for several weeks or more can cause lid drooping in the treated eye. Most all of these get better with stoping the drop(s) . Consult your eye specialist if it does not or if vision is getting blurred also.