Way back when I used to pump hair conditioner up my ass to emulate cum. At first it felt pretty good, but after like two or three consecutive days of that my asshole would end up really dried out. So much so to the point where inserting anything was painful. Could be hair conditioner in this case.
It could be that she's not properly lubed down there. Staying hydrated, staying away from meats and eats lots of veggies are key for anal lubrication. Eating surgery candy can help too. And just not tightening up much.