It could have a scratch they are sometimes hard to see. My mare just went through a scratched eye...it took weeks for it to heal...its just now getting back to normal. I had her on bute for pain/swelling and a ointment for 3/4 weeks twice a day of course I took her to the vet. I am broke as well...but your better off taking them to a vet than waiting and have it get worse. Do you have pictures of her eye?
A fly mask is a good idea, also put duck tape or something over the part where her eye is that way it keeps the sun out as well as the dirt

Beings you said shes sensitve to light(my mare was as well which is why I think it may have a hairline scratch) Just a idea

I agree...dont start putting anything in it until you talk to a vet. How long has it been watering?