Eye lid drop after cataract operation

it is not uncommon to have a droopy lid after any type of eye surgery:  cataract, glaucoma, retina.  When the surgery is done, a speculum is placed to hold the lids open.  This may cause some stretching of the muscles so that the lid will appear lower.  The lid may slowly come back up to its' normal position. So give it more time.  If it does not after about 6 months you may want to have it surgically lifted.      

We can't tell you why this occurs.  There are many theories.  Generally because this can recover, it is best to give this 6 to 12 months to resolve on its own before deciding that you need to do something about it.

Ptosis can occur with almost any eye surgery but is uncommon, usually mild and self limited. 
Sometimes it can occur when there is inflammation that isn't completely treated after surgery or relapses after you stop drops.Sometimes it occurs because eye drops are irritating. I wouldn't recommend altering your drop post op without consulting your doctor.
It's easy to fix with ptosis repair.