Ask an Eye Doctor and Get an Answer ASAP
I don't think you have any choice if she is unable to tell you what kind of pain or whether her vision is affected or not. This could be something very benign like a subconjunctival hemorrhage (from blood thinner) to iritis to acute glaucoma. And if you are just the caretaker and not her custodian, you would also be taking a legal risk in not taking her to see a doctor.
Hope this helps and good luck.