Does she have a litter box in the house? It is possible being an outdoor cat something has happened to scare her when she is outside. Have you thought about trying to make her an indoor only with the boxes set up as MBIE suggested?
Assumptions do nothing but make an ass out of u and me.
We can stick our heads in the sand for only so long before it starts choking us. Face it folks. The pet population is bad ALL OVER THE WORLD!