I would like some extra history. When was she operated for Cataract
.Is there history of any recent trauma.
But with as much history you have given: Red eye with smaller looking eye & severe pain
suggests of either the cornea being involved or deeper reaction(like uveitis).
She requires proper ophthalmological examination in an eye hospital.
You can start Moxifloxacin eye drop
1drop hourly till you reach the hospital.